Extend registration pages

- [x] Redirect to a "registration succesful" page
- [x] Mind the capacity of access levels
- [x] Dynamically add multiple tickets (for friends)
- [ ] Add price and "publicly avaible" to access levels and use this in the registration view
- [ ] Ask for student number of person paying or enforce them to login via CAS
- [ ] Ask for student number per ticket (no 2 tickets with same student number) to allow entry to event via student card