Resolve "Deleting a user shouldn't delete associated records"

What does this MR do?

Supplement #12726 (closed)/!7393 (merged) by migrating a user's merge requests, notes, award emoji, and abuse reports to the ghost user before the user is destroyed.

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

Tasks

• Implementation
  • Refactor / general cleanup
  • Migrate
    • Merge requests
    • Notes
    • Abuse reports
      • When User A reports an "abuse report" against User B, and User A is blocked, the abuse report's target changes to User A. Why? (#30514 (closed))
    • Award emoji
      • Is this possible with the current ghost user implementation?
• Tests
  • Added
  • Passing
• Verification
  • Make sure no uniqueness validations are going to conflict with the ghost user
    • Award emoji
    • Merge requests
    • Notes
    • Abuse reports
  • Verify from the UI
    • Merge requests
    • Notes
      • Merge request
      • Issue
      • Snippet
    • Abuse reports
    • Award emoji
      • Make sure the award emoji uniqueness change doesn't break anything in the UI
        • Snippet
        • Note
          • Merge request
          • Issue
          • Snippet
        • Issues
        • Merge requests
• Meta
  • CHANGELOG entry created
  • API support added
  • Branch has no merge conflicts with master
  • Squashed related commits together
  • Added screenshots
  • Documentation added/updated
  • Check for clean merge with EE
    • Create EE branch
    • Create EE merge request
    • Make sure ee_compat_check passes
• Review
  • Miniboss
  • Endboss
• Wait for merge

What are the relevant issue numbers?

• Closes #28695 (closed)
• Closes #30514 (closed)
• EE merge request to resolve conflicts: gitlab-org/gitlab-ee!1581