Improved note selection and user permissions

* Improved note selection: Users choose their shows and corresponding timeslots are loaded into the select via ajax (improvement still necessary to avoid queryset when initially loading)
See #17

* Users can only see and edit their shows, timeslots and notes
* Users can only edit their own profiles

* Changed fallback_playlist to fallback_playlist_id
* Timelots inherit the is_repetition flag from their schedules when created
* Added field 'memo' to timeslots (for internal notes)
See #21