Python 3.8 is minimal version now

Close #52

- 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')

Close #49

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

Also clean-up unused imports.

Also clean-up unused imports.

Also clean-up unused imports.

Public Shows

See merge request !9

- Django==1.11.29
- django-cors-headers==3.2.1
- django-oidc-provider==0.7.0
- django-tinymce==2.8.0
- django-versatileimagefield==1.11
- djangorestframework==3.11.0
- drf-nested-routers==0.91
- Pillow==4.3.0
- python-dateutil==2.8.1
- PyYAML==3.13

Later, upgrade
- Django up to 2.2.11, the current LTS version
- django-versatileimagefield up to 2.0, the current stable version
- Pillow up to 7.0.0, the current stable version
- PyYAML up to 5.3.1, the current stable version