Add event registration
Closes #20 (closed)
Adds the functionality to register for events in the app.
Steps to use this feature
- Open an event with registrations (but no extra fields) in the app.
- Press the "Aanmelden" button.
- Notice that you appear on the registration list.
- Press the "Afmelden" button.
- Notice that you disappear from the registration list.
- Open an event with registrations and extra fields in the app.
- Press the "Aanmelden button".
- Notice it opens a screen with the extra fields.
- Enter the fields and press "Aanpassen".
- Notice that the event screen shows an "Aanmelding bijwerken" button.