[SIEM] New Overview Page (elastic#54783)

## [SIEM] Overview Page "1.5"

A redesigned SIEM Overview page that includes `Recent timelines`, a `Security news` feed, visualizations, and rolled-up event counts

![overview-day](https://user-images.githubusercontent.com/4459398/72396016-90f53600-36f8-11ea-9b41-6d54d09de589.png)

![overview-night](https://user-images.githubusercontent.com/4459398/72394575-fb57a780-36f3-11ea-868e-8fcd2c5c4543.png)

### Overview enhancements
- Added the global Search bar and Date picker to the Overview page
- New `Recent timelines` widget affords quick access to favorite and recently modified timelines
- New `Security news` widget
- New Kibana advanced settings (toggle switch) for enabling or disabling the news widget and configuring the news URL
![news-settings](https://user-images.githubusercontent.com/4459398/72362776-fd4c4700-36b0-11ea-805b-3c7353f2c1cd.png)
- New `Events count by dataset` widget
- Updated the `Host Events` and `Network Events` widgets to integrate with the Search bar and date picker input
- Enhanced the `Host Events` and `Network Events` widgets to use an accordion paradigm that summarizes stats by source (e.g. `Auditbeat`, `Endgame`)
- Enhanced the `Host Events` and `Network Events` widgets to visualize relative percentages of events collected as progress bars
- New `Alerts count by category` widget
- New `Signals count by MITRE ATT&CK™ category` widget
- New `View events`, `View alerts`, and `View signals` navigation buttons for their respective visualizations


### FTUE enhancements
- FTUE "no data" view design refresh
![ftue](https://user-images.githubusercontent.com/4459398/72361771-43a0a680-36af-11ea-969f-5872ac4a01a1.png)
- When the FTUE "no data" page is displayed, hide all global navigation links (i.e. `Hosts`, `Network`, `Detection engine`), such that only `Overview` appears in the global nav
- App Help popover design refresh
![help](https://user-images.githubusercontent.com/4459398/72362132-d80b0900-36af-11ea-9b58-1fd3b923b7c8.png)
- Removed the `Beta` badge and `Security Information & Event Management with the Elastic Stack` from the Overview header

- Tested in Chrome `79.0.3945.117`, Firefox `72.0.1`, and Safari `13.0.4`

## Known issues

- The `siem:newsFeedUrl` advanced setting is defaulted to `https://feeds.elastic.co/kibana`
- The `Signals count by MITRE ATT&CK™ category` visualization does not display all categories
- The `Signals count by MITRE ATT&CK™ category` visualization may require a different index pattern
- `EuiButtonGroup` throwing a `Can't perform a React state update on an unmounted component` warning when switching from the Overview tab

https://github.com/elastic/siem-team/issues/484

docs/development/core/public/kibana-plugin-public.doclinksstart.links.md

@@ -79,7 +79,10 @@ readonly links: {
             readonly introduction: string;
         };
         readonly kibana: string;
-        readonly siem: string;
+        readonly siem: {
+            readonly guide: string;
+            readonly gettingStarted: string;
+        };
         readonly query: {
             readonly luceneQuerySyntax: string;
             readonly queryDsl: string;

docs/development/core/public/kibana-plugin-public.doclinksstart.md

@@ -17,5 +17,5 @@ export interface DocLinksStart
 |  --- | --- | --- |
 |  [DOC\_LINK\_VERSION](./kibana-plugin-public.doclinksstart.doc_link_version.md) | <code>string</code> |  |
 |  [ELASTIC\_WEBSITE\_URL](./kibana-plugin-public.doclinksstart.elastic_website_url.md) | <code>string</code> |  |
-|  [links](./kibana-plugin-public.doclinksstart.links.md) | <code>{</code><br/><code>        readonly filebeat: {</code><br/><code>            readonly base: string;</code><br/><code>            readonly installation: string;</code><br/><code>            readonly configuration: string;</code><br/><code>            readonly elasticsearchOutput: string;</code><br/><code>            readonly startup: string;</code><br/><code>            readonly exportedFields: string;</code><br/><code>        };</code><br/><code>        readonly auditbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly metricbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly heartbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly logstash: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly functionbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly winlogbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly aggs: {</code><br/><code>            readonly date_histogram: string;</code><br/><code>            readonly date_range: string;</code><br/><code>            readonly filter: string;</code><br/><code>            readonly filters: string;</code><br/><code>            readonly geohash_grid: string;</code><br/><code>            readonly histogram: string;</code><br/><code>            readonly ip_range: string;</code><br/><code>            readonly range: string;</code><br/><code>            readonly significant_terms: string;</code><br/><code>            readonly terms: string;</code><br/><code>            readonly avg: string;</code><br/><code>            readonly avg_bucket: string;</code><br/><code>            readonly max_bucket: string;</code><br/><code>            readonly min_bucket: string;</code><br/><code>            readonly sum_bucket: string;</code><br/><code>            readonly cardinality: string;</code><br/><code>            readonly count: string;</code><br/><code>            readonly cumulative_sum: string;</code><br/><code>            readonly derivative: string;</code><br/><code>            readonly geo_bounds: string;</code><br/><code>            readonly geo_centroid: string;</code><br/><code>            readonly max: string;</code><br/><code>            readonly median: string;</code><br/><code>            readonly min: string;</code><br/><code>            readonly moving_avg: string;</code><br/><code>            readonly percentile_ranks: string;</code><br/><code>            readonly serial_diff: string;</code><br/><code>            readonly std_dev: string;</code><br/><code>            readonly sum: string;</code><br/><code>            readonly top_hits: string;</code><br/><code>        };</code><br/><code>        readonly scriptedFields: {</code><br/><code>            readonly scriptFields: string;</code><br/><code>            readonly scriptAggs: string;</code><br/><code>            readonly painless: string;</code><br/><code>            readonly painlessApi: string;</code><br/><code>            readonly painlessSyntax: string;</code><br/><code>            readonly luceneExpressions: string;</code><br/><code>        };</code><br/><code>        readonly indexPatterns: {</code><br/><code>            readonly loadingData: string;</code><br/><code>            readonly introduction: string;</code><br/><code>        };</code><br/><code>        readonly kibana: string;</code><br/><code>        readonly siem: string;</code><br/><code>        readonly query: {</code><br/><code>            readonly luceneQuerySyntax: string;</code><br/><code>            readonly queryDsl: string;</code><br/><code>            readonly kueryQuerySyntax: string;</code><br/><code>        };</code><br/><code>        readonly date: {</code><br/><code>            readonly dateMath: string;</code><br/><code>        };</code><br/><code>    }</code> |  |
+|  [links](./kibana-plugin-public.doclinksstart.links.md) | <code>{</code><br/><code>        readonly filebeat: {</code><br/><code>            readonly base: string;</code><br/><code>            readonly installation: string;</code><br/><code>            readonly configuration: string;</code><br/><code>            readonly elasticsearchOutput: string;</code><br/><code>            readonly startup: string;</code><br/><code>            readonly exportedFields: string;</code><br/><code>        };</code><br/><code>        readonly auditbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly metricbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly heartbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly logstash: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly functionbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly winlogbeat: {</code><br/><code>            readonly base: string;</code><br/><code>        };</code><br/><code>        readonly aggs: {</code><br/><code>            readonly date_histogram: string;</code><br/><code>            readonly date_range: string;</code><br/><code>            readonly filter: string;</code><br/><code>            readonly filters: string;</code><br/><code>            readonly geohash_grid: string;</code><br/><code>            readonly histogram: string;</code><br/><code>            readonly ip_range: string;</code><br/><code>            readonly range: string;</code><br/><code>            readonly significant_terms: string;</code><br/><code>            readonly terms: string;</code><br/><code>            readonly avg: string;</code><br/><code>            readonly avg_bucket: string;</code><br/><code>            readonly max_bucket: string;</code><br/><code>            readonly min_bucket: string;</code><br/><code>            readonly sum_bucket: string;</code><br/><code>            readonly cardinality: string;</code><br/><code>            readonly count: string;</code><br/><code>            readonly cumulative_sum: string;</code><br/><code>            readonly derivative: string;</code><br/><code>            readonly geo_bounds: string;</code><br/><code>            readonly geo_centroid: string;</code><br/><code>            readonly max: string;</code><br/><code>            readonly median: string;</code><br/><code>            readonly min: string;</code><br/><code>            readonly moving_avg: string;</code><br/><code>            readonly percentile_ranks: string;</code><br/><code>            readonly serial_diff: string;</code><br/><code>            readonly std_dev: string;</code><br/><code>            readonly sum: string;</code><br/><code>            readonly top_hits: string;</code><br/><code>        };</code><br/><code>        readonly scriptedFields: {</code><br/><code>            readonly scriptFields: string;</code><br/><code>            readonly scriptAggs: string;</code><br/><code>            readonly painless: string;</code><br/><code>            readonly painlessApi: string;</code><br/><code>            readonly painlessSyntax: string;</code><br/><code>            readonly luceneExpressions: string;</code><br/><code>        };</code><br/><code>        readonly indexPatterns: {</code><br/><code>            readonly loadingData: string;</code><br/><code>            readonly introduction: string;</code><br/><code>        };</code><br/><code>        readonly kibana: string;</code><br/><code>        readonly siem: {</code><br/><code>            readonly guide: string;</code><br/><code>            readonly gettingStarted: string;</code><br/><code>        };</code><br/><code>        readonly query: {</code><br/><code>            readonly luceneQuerySyntax: string;</code><br/><code>            readonly queryDsl: string;</code><br/><code>            readonly kueryQuerySyntax: string;</code><br/><code>        };</code><br/><code>        readonly date: {</code><br/><code>            readonly dateMath: string;</code><br/><code>        };</code><br/><code>    }</code> |  |