fix: 1154 Add LOGOUT_REDIRECT_URL setting

[blag]

Description

This pull request addresses #1154, by redirecting to LOGOUT_REDIRECT_URL in the /logout view if LOGOUT_REDIRECT_URL is set in the config, falling back to self.appbuilder.get_url_for_index.

It also adds LOGOUT_REDIRECT_URL to the Configuration keys table in docs/config.rst, and adds a unit test.

This PR is the exact same changes as #1160, but reopened since that one didn't seem to garner any feedback or interest once it was closed by the stale bot. This PR will cleanly merge, and will likely also solve apache/airflow#17279, so there is already people waiting on this fix to be merged.

ADDITIONAL INFORMATION

• Has associated issue: fixes Make redirect after logout configurable #1154 (which was automatically closed by the stalebot because it was "stale", even though the issue was not fixed)
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1749 (c5fb3d2) into master (319b6be) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1749   +/-   ##
=======================================
  Coverage   76.88%   76.88%           
=======================================
  Files          56       56           
  Lines        8136     8136           
=======================================
  Hits         6255     6255           
  Misses       1881     1881           

Flag	Coverage Δ
python	76.88% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask_appbuilder/security/views.py	62.16% <100.00%> (ø)

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[blag]

Commenting so the stalebot doesn't mark this as stale. This PR is in a mergable state, it just needs a review.
@dpgaspar - Are you going to have time to review and merge this? Or do you need some additional help managing this project?

[blag]

Thank you!