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.

- Reorder the code inside the methods to fail fast on autorization,
- Replace `int_or_none` with a more generic solution, and move to utils,
- Add `get_values` and move `pk_and_slug` as `get_pk_and_slug` into utils,
- Replace calls to static methos in models local queries,
- Return meaningful status code while creating and updating resources,
- Return `409` when creating or updating a schedule produces a conflict.

Close #51

See #21 #23

See #8