Fix/register button shown after late cancellation

Closes #72

Previous behaviour

The event detail shows the "Register" button and some wrong text when the user has cancelled after the cancellation deadline.

Steps to reproduce:

1. Register for an event.
2. Wait until the cancellation deadline has passed.
3. Cancel your registration.
4. Notice the app either displays the text "Cancellation is not possible anymore" (when the registration deadline has passed), or it displays the "Register" button (when the registration deadline has not yet passed).
5. Notice the app crashes when the "Register" button is pressed.

New behaviour

The event detail displays the correct text and hides the "Register" button when needed.

Steps to validate that it works:

1. Register for an event.
2. Wait until the cancellation deadline has passed.
3. Cancel your registration.
4. Notice the app displays the text "Registration is not allowed anymore, as you cancelled your registration after the deadline" when the registration deadline has not yet passed, and the text "Cancellation is not possible anymore" is no longer displayed after the registration deadline.