Developer changes

- Added "developer options" to manage users, view statistics (not in
yet), and manipulate the "sqltutor" database.
- Changed tutorial setup to "admin options"
- Added "developer" and "admin" types to userbean.
- Logging out now sends the user to the homepage.
- All access redirects now result in a 404-Error.
- Added "admin codes", which will be used by students to connect to a
teacher's tutorials. Linked admin codes will decide which tutorials are
visible.