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)