Change paid boolean value for registration to enum field to support registration of pin payments

Closes #457 (closed)

Short description

Before we could only register if a member paid for an event but not differentiate between cash and card payments. This is now possible.

Steps to use this feature

1. Create an event
2. Add registrations
3. Notice the radio buttons in the event registrations admin view
4. Notice the dropdown box in de registration admin form