WIP: Persist published state for status page issues

What does this MR do?

Keeps status_page_published_incidents table up to date with publishing/unpublishing issues. Does not change any logic by which an issue is published or unpublished.

Related issue: #209849 (closed)

MRs for full feature:

• !29994 (merged) - Adds status_page_published_incidents table
• !30120 (closed) - Saves records to table when issues are published/unpublished
• TBD - Disable auto-publishing issues & Add /publish command

Migration Output:

$ rake db:migrate
== 20200421195234 BackfillStatusPagePublishedIncidents: migrating =============
== 20200421195234 BackfillStatusPagePublishedIncidents: migrated (0.1096s) ====

$ rake db:migrate:down VERSION=20200421195234
== 20200421195234 BackfillStatusPagePublishedIncidents: reverting =============
== 20200421195234 BackfillStatusPagePublishedIncidents: reverted (0.0000s) ====

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team