Add event registration

Closes #20

Short description

Adds the functionality to register for events in the app.

Steps to use this feature

1. Open an event with registrations (but no extra fields) in the app.
2. Press the "Aanmelden" button.
3. Notice that you appear on the registration list.
4. Press the "Afmelden" button.
5. Notice that you disappear from the registration list.
6. Open an event with registrations and extra fields in the app.
7. Press the "Aanmelden button".
8. Notice it opens a screen with the extra fields.
9. Enter the fields and press "Aanpassen".
10. Notice that the event screen shows an "Aanmelding bijwerken" button.