[Maps] Make maps shareable across Spaces

[nickpeihl]

As a precursor to #167901, we think it might be best to make maps shareable across spaces.

Maps was made "share-capable" in 8.0. To make Maps completely "shareable" we need to complete the steps in Phase 2 of the "Sharing saved objects" documentation.

However, there are a few issues not covered by the documentation.

Related objects such as data views are automatically shared to other spaces when sharing a Map from the Saved Objects management. However, a user can still change or add a data view to an existing Map where the data view is not available in the other spaces. We have a similar issue for dashboards.

I think, ideally, we would warn the user immediately if the data view they are adding is not available in other spaces. We could prepend a warning icon to the option in the dropdown or we could disable the option completely.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[nreese]

Closing, issue iceboxed in #188573