- `HostLinkSerializer` is a `serializers.ModelSerializer` again.

- fixed image field names,
- removed fields that are no longer used

The following attribute names were marked as deprecated in the schedule
request and have been replaced by their new names:

  dstart → first_date
  until → last_date
  tstart → start_time
  tend → end_time
  byweekday → by_weekday

Apart from that this commit fixes a few documentation errors.

When making the serialized fields explicit in
0ee3a655 we forgot to add the id field
that is used in various places.

This adds extensive API documentation based on the official API
documentation [1] and conflict resolution [2] documents.

Where possible field documentation was added to models or serializers,
so that other code like auto-generated forms can also profit from these
changes (hence the migration part of this commit).

The changes introduce two new API endpoints.

`/api/v1/schema/` exposes the API schema as an OpenAPI 3.0.3 document.
The standard format is yaml but can be switched to JSON by appending the
`?format=json` query parameter.

`/api/v1/schema/swagger-ui/` renders a visual representation of the
OpenAPI 3 specification with support for testing the individual API
endpoints including authentication.

[1]
https://gitlab.servus.at/aura/meta/-/blob/ec3c753d34ccb0269969808ac7dc28fff2ff1648/docs/development/api-definition.md
[2]
https://gitlab.servus.at/aura/meta/-/blob/ec3c753d34ccb0269969808ac7dc28fff2ff1648/docs/development/conflict-resolution.md

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

This adds extensive API documentation based on the official API
documentation [1] and conflict resolution [2] documents.

Where possible field documentation was added to models or serializers,
so that other code like auto-generated forms can also profit from these
changes (hence the migration part of this commit).

The changes introduce two new API endpoints.

`/api/v1/schema/` exposes the API schema as an OpenAPI 3.0.3 document.
The standard format is yaml but can be switched to JSON by appending the
`?format=json` query parameter.

`/api/v1/schema/swagger-ui/` renders a visual representation of the
OpenAPI 3 specification with support for testing the individual API
endpoints including authentication.

[1]
https://gitlab.servus.at/aura/meta/-/blob/ec3c753d34ccb0269969808ac7dc28fff2ff1648/docs/development/api-definition.md
[2]
https://gitlab.servus.at/aura/meta/-/blob/ec3c753d34ccb0269969808ac7dc28fff2ff1648/docs/development/conflict-resolution.md

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