This reverts merge request !42

This filter is currently applied programmatically in the dashboard. It
makes much more sense to do this on a database level.

refs dashboard#246

These filters now support datetimes as well as dates and their naming
now reflects what they are querying.

The default values break some valid use cases and there’s no real value
in applying them by default.

refs #189

The filter annotation for ordering by show ownership introduced
a join that caused some results to appear more than once.

The dashboard must be able to sort shows.

In aa7e7f41 filters were renamed
to a camelCase scheme. This is actually wrong, because the filter
names are already transformed to snake_case when the are applied
to the FilterSet.

The nested timeslot routes allowed us to filter the timeslots by show
and schedule. We’ve deprecated nested endpoints with the alpha.2
release, but still need the actual filtering logic.

To be honest I'm not sure this is the right way to do it.

This is mainly so we can start phasing out nested routes.
With nested routes objects were filtered based on route parameters.
Therefore we need something similar in the form of query parameters.

Fixes: #133

If a collection is filtered by an invalid id (meaning the model the
query argument points to with that specific id does not exist), we no
longer return an error. Instead the filter is applied without a lookup.

Fixes #104

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