Remove action runners on user deletion (#27902) (#27908)

Remove action runners on user deletion (#27902) (#27908)

Backport #27902 by @earl-warren

- On user deletion, delete action runners that the user has created.
- Add a database consistency check to remove action runners that have
nonexistent belonging owner.
- Resolves https://codeberg.org/forgejo/forgejo/issues/1720

(cherry picked from commit 009ca7223dab054f7f760b7ccae69e745eebfabb)

Co-authored-by: Earl Warren <109468362+earl-warren@users.noreply.github.com>
Co-authored-by: Gusted <postmaster@gusted.xyz>