- updated the fixture
- updated the serializer
- updated some views

- Introduce `ActiveInactiveMixin` with the `get_queryset` method and `permission_classes`
- Use `ActiveInactiveMixin` for `Category`, `FundingCategory`, `Language`, `MusicFocus`, `Topic`, `Type` and `Host`

Close #51

Python 3.8 is minimal version now

- 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')

- Add missing Serializer,
- also clean-up unused imports.

See #28

- 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.

Fixes #26

Fixes #27