cba_id was always part of the serialized note representation through the
use of `fields = "__all__"``and shouldn’t be write-only.

There were some problems during audio_url generation for notes:

1. the cba_id was never part of the Note serializer so getting
   values from the validated data needed to fail.
2. the get_audio_url utililty function returned None if
   no cba_id was provided if a the CBA_API_KEY was missing,
   but None but the respective model field is not nullable on the note.
3. Request or JSON decoder errors would bubble unhandled, even though
   audio_url generation seems to be optional and should not necessarily
   interfere with note creation.

- update the model
- migrate the model
- update the serializer
- update the views

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

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.

Fixes #25

+
* 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