Clean-up code

- 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')