Add support for tracking when an issue is published/unpublished

What does this MR do?

#209849 (closed) will add /publish quick action. We will want a convenient way to track when an issue is published to the status page. This will be kept in the status_page_published_incidents table, added in !29994 (merged).

This MR adds methods to make tracking published issues easier. They should help with these two use cases:

• When an issue is published, a StatusPage::PublishedIncident is created.
• When an issue is unpublished, the corresponding StatusPage::PublishedIncident is deleted.

Adding the quick action is a pretty big MR, so separating this functionality out should make it easier to review. Note that these methods are not yet used by any app code.

As these changes are entirely non-user facing, I'm going to skip the changelog.

Related issue: #209849 (closed)

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