Allow the creation of recurring events (as copies)
The event form includes the additional fields "recurrence period" and "count". These fields are evaluted after form submission.
Open details:
- The implementation ("_create_time_based_repetitions") is a partial copy of "save". This is hard to avoid, since "save" is intertwined with self.instance and thus does not easily allow the creation of multiple objects.
- Notifications are sent to subscribers only for the first new event (not for its later repetitions).
- The form field for the number of repetitions should only be visible, if the form field for the repetitions period does not contain "once".
- All new items use the same title and text. Maybe some placeholders would be nice. But such a feature would be new for grouprise.
Closes: #527 (closed)