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

Simply omitting properties in the API in certain edge cases makes API
client implementations brittle and documentation a lot harder.

This gives the calling code better control over the handling of such
errors and makes it easy to differentiate between errors and valid
return types.

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

Simply omitting properties in the API in certain edge cases makes API
client implementations brittle and documentation a lot harder.

This gives the calling code better control over the handling of such
errors and makes it easy to differentiate between errors and valid
return types.

This adds a string representation for all program models except the
schedule so that it is easier to identify objects in DRF’s filter,
create, and update forms.

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

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