MR unmergeable notification

What does this MR do?

• Notify MR author and merge user when MR became unmergeable.
• Todo: create TODO for MR author and merge user when MR became unmergeable
• User should not receive notification if his/her notifications are disabled for the project.

Are there points in the code the reviewer needs to double check?

As this is a new type of notification, a new type of NotificationRecipientService is added. Please see if this is okay.

Why was this MR needed?

To let MR author know about conflicts early.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#22041 (closed)

ping @smcgivern