This commit adds several missing "cascade delete" actions to relationships on database tables. This effectively fixes errors while trying to delete user accounts because of pending child records.
Additionally, the observers for applications and vacancies were removed, since they are now obsolete.
The account deletion system was also refactored.
This commit rewrites the previously broken auto ban cleaner. It's now much more efficient and compact, and it removes bans correctly and on the right date.
Fixes#1
