- Add type annotations,
- Cleanup logic,
- utils.parse_time now accepts to time format strings.

- remove ThumbnailsMixin and thumbnails fields,
- remove image_* fields and theis usage,
- introduce utils.delete_links function

Most of the queryset filtering for subroutes formerly implemented
through custom get_queryset methods can happen semi-automated through a
mixin, making the behaviour re-usable and allowing us strip a lot of
additional logic for retrieving objects.

The use of the viewset get_object method also allows us to implement
object access with permission classes in the future, and should make a
whole lot of other code obselete.

This change is primarily motivated by the fact that documentation of the
API will become a lot easier if the subroutes don’t impose arbitrary
restrictions on the specific subpath under which an operation is
allowed. For instance updating a schedule should be possible through
either /schedules/<pk>/ or
/shows/<show_pk>/schedules/<schedule_pk>/ as long as all the necessary
data is present in the request body.

The show retrieve method allowed shows to be identified through the slug
or the id. This is handy, but was restricted to the retrieve and did not
apply to the update nor delete methods. The proper way to implement this
kind of behaviour is through overriding get_object so that route
identifiers are handled consistently.

Most of the queryset filtering for subroutes formerly implemented
through custom get_queryset methods can happen semi-automated through a
mixin, making the behaviour re-usable and allowing us strip a lot of
additional logic for retrieving objects.

The use of the viewset get_object method also allows us to implement
object access with permission classes in the future, and should make a
whole lot of other code obselete.

This change is primarily motivated by the fact that documentation of the
API will become a lot easier if the subroutes don’t impose arbitrary
restrictions on the specific subpath under which an operation is
allowed. For instance updating a schedule should be possible through
either /schedules/<pk>/ or
/shows/<show_pk>/schedules/<schedule_pk>/ as long as all the necessary
data is present in the request body.

The show retrieve method allowed shows to be identified through the slug
or the id. This is handy, but was restricted to the retrieve and did not
apply to the update nor delete methods. The proper way to implement this
kind of behaviour is through overriding get_object so that route
identifiers are handled consistently.

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