Allow users to be hard-deleted from the admin panel

What does this MR do?

Expose the "hard delete" user function in the admin panel. This allows a user, their contributions and any groups solely owned by that user to be removed all at once.

This functionality already exists for users reported for abuse or spam; however, it's useful to administrators more generally.

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

Why was this MR needed?

Screenshots (if relevant)

User show page:

User list page:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28694 (closed)