Close #51

- Make `Host.is_editable`, `Show.is_editable`, `Note.is_editable`, and
  `Note.get_audio_url` static methods,
- Make `Schedule.instantiate_upcoming`, `Schedule.generate_timeslots`,
  `Schedule.get_collisions`, `Schedule.generate_conflicts`,
  `Schedule.make_conflicts` and `Schedule.resolve_conflicts` static
  methods. Most of these methods do not belong here.
- Fix signatures of methods for subclasses of `viewsets.ModelViewSet`
- Fix doctrins for subclasses of `viewsets.ModelViewSet`
- Fix comparisons with None (replace '==' and '!=' with 'is' and 'is not')

Also clean-up unused imports.

- Field "FundingCategory" is not mandatory anymore
- Added new field "is_active": Active shows will be determined either by existing timeslots in the future (if show is active). If show is not active it will always be considered as inactive, even if there are upcoming timeslots.

+
* Fixed bug when deleting a note
* Extended schedule list view and filters in admin ui
* Got rid of unnecessary scopes
* Rearranged settings

* for adding a number of (business) days to the schedule (e.g. "on the rrules' following day")
* fixed a bug for monthly recurrences
* and some minor changes

See #8

* added corsheaders middleware to work properly
* fixed bug when updating a timeslot
* after updating a timeslot, its repetition will be returned if there's one
* included field 'ppoi' in shows

See #22 #23

See #21 #23

and:
* Set correct login redirect url for oidc
* Added django-cors-headers for cross-domain api requests

See #21 #22

* Properly generate thumbnails for shows, hosts and notes
* Return thumbnails in API

See #22

...as well as some minor changes and bug fixes.

See #8 #10 #22 #24

...and filters for admin ui and api.

See #21

* Added dryrun
* Added reassignment of notes and playlists

See #8

See #8 #10 #20 #22

Documentation: https://gitlab.servus.at/autoradio/meta/blob/master/conflict-resolution.md

* Users can see and change all notes and hosts of shows they own - even if they were created by another show owner
* A host can be assigned to a note (if host is editable)
* Added django-oidc-provider app
* Added limit/offset pagination to endpoints /shows, /timeslots and /notes
* Common users may not change the show's name anymore via API
* Playout API can be called with start (date) and end (date) parameters
* Added filters to endpoints:

/shows/?host=1  Retrieves shows of a given host
/shows/?owner=1 Retrieves shows of a given owner (= user ID)
/notes/?host=1  Retrieves notes assigned to a host
/notes/?owner=1 Retrieves notes a user has created (= user ID)

Had to set settings const USE_TZ (= reflect timezone in mysql datetimes) to True for OIDC to work with mysql.

Don't know yet if that's a problem.

See #22 #23

See #21