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 makes optional registration parameters optional; They should only be filled in after the user has submitted an application. The application will crash during registration without these parameters being optional, and adding them to the form would be illogical.
