First calendar addition doesn't refresh the list (#75)

* Reinitializing `CalendarModel` listeners on resume

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* CalendarListActivity: Start watching calendars as soon as permissions are granted

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>