Deleting a user shouldn't delete associated records
Description
- When a user is deleted, all the user's data (except issues - #12726 (closed)/!7393 (merged)) are deleted.
Proposal
- Move a user's merge requests, notes, award emoji, and abuse reports to the "Ghost User" prior to deletion.
Links / references
- Discussed in original ghost user merge request: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7393#note_24100023
- Related issue (allow turning off the ghost user feature): #28694 (closed)