Add event view in calendar

[thellmund]

The calendar functionality should be aligned with the Android version, namely:

1. Allow the user to see details on a calendar event when tapping on it. This should also allow the user to open the room finder for events with a location.
2. Delete existing calendar event.
3. Edit existing calendar event.
4. Add new calendar event.

[kordianbruck]

This issue is too big. Please split it up. Not sure that we currenlty can delete events.

[thellmund]

I will split the tasks into multiple issues. This issue was more of a brain-dump on my part. Deleting events is possible in the Android app.

[AW-tum]

Event info already implemented
All Calendar Customization is handled in #239