This commit addresses an issue where users could submit as many applications as they wanted by simply navigating to the previous page and resubmitting the form, therefore bypassing validation that was only existent in the front end.
Fixes#20.
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.
