Add a "full_user_purge" method

[db0]

Purging a user doesn't purge their communities. Might be good to give a method for admins who want to do that. The method should look up all user-owned communities and purge them, then finally purge the user

"full_user_purge" is a provisional name. Feel free to use something else. Maybe "bury", "banish" or something cool :D