Use it with `HostSerializer`, `NoteSerializer` and `ShowSerializer`

- reorder explicitily named fields,
- define `{created,updated}_{at,by}` as read-only-fields

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

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

- `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request and have been replaced by their new names: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