[AIRFLOW-5693] Support the "blocks" component for the Slack messages

[mingrammer]

Make sure you have checked all steps below.

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• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
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Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

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[mingrammer]

CI failed due to this issue. (docker/for-linux#830)

And, this issue seems to be fixed. Please re-run the CI :) (docker/for-linux#830 (comment))

Err:13 https://download.docker.com/linux/debian stretch/stable amd64 Packages
  Writing more data than expected (11116 > 9983)
  Hashes of expected file:
   - Filesize:9983 [weak]
   - SHA512:4faf843dcb2544a9135a36ecfc4b914dd4e9cdada9a5e20e4994f5dd33d2d5aacb44f237bf9bf7048bac571e8be47d04975f8d93a19dab67dbfc231b68012011
   - SHA256:db9d0a054919cfa52440f1d59ec3f4f2fd15b89661c87f35f02a24f3832a6ce6
   - SHA1:d52b8071d1449718bc483c54a5bc698a26107aa6 [weak]
   - MD5Sum:d7b024e64ef3849c725f4ce1f05bf7a7 [weak]
  Release file created at: Sat, 17 Aug 2019 05:04:49 +0000

[feluelle]

@mingrammer do you mind fixing another doc issue in it as well? 😀

See: https://airflow.readthedocs.io/en/latest/_api/airflow/contrib/hooks/slack_webhook_hook/index.html#airflow.contrib.hooks.slack_webhook_hook.SlackWebhookHook._build_slack_message

There is missing a blank line between the title and the param section in the _build_slack_message.

And this:

- :return: Slack message (str) to send
+ :return: Slack message to send
+ :rtype: str

After that I am happy to merge it when everything is green :)

[mingrammer]

I fixed all docstring issues in this file!

[codecov-io]

Codecov Report

Merging #6364 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #6364      +/-   ##
=========================================
- Coverage   80.41%   80.1%   -0.31%     
=========================================
  Files         616     616              
  Lines       35805   35808       +3     
=========================================
- Hits        28791   28683     -108     
- Misses       7014    7125     +111

Impacted Files	Coverage Δ
airflow/contrib/hooks/slack_webhook_hook.py	95.83% <100%> (+0.27%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️

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[feluelle]

Nice!