fix: raise DRF-compatible exception for configuration errors

See merge request !55

ValueError has no built-in handling in DRF causing it to return an HTML
error page. This is unfortunate for clients that expect a JSON error
object like the dashboard.

fix(program-generation): fix undefined behaviour around start and end query parameters

See merge request !54

ids-filter for show, schedule, and timeslot endpoints

See merge request !53

The radio program is a continuous stream of program slots. Clients can
expect that the program generated for their queries fit the start and
end parameters given by the client. However, they *cannot* expect the
actual radio program (in the form of Timeslot objects) to fit these
artifical boundaries.

As the program endpoint should always output the actual program
broadcasted by the radio we must include timeslots that
  * have started before the specified start query but end after it
  * or end after the specified end query but start before it.

The changes in this commit ensure that the program matches the given
range exactly and that planned timeslots in that range are always
included.

This makes it easy to list entries with specific ids, which comes in
handy when clients want to fetch multiple known objects at once.

build: update Poetry dependencies: coverage, django-cors-headers, djangorestframework, flake8, jsonschema, pillow, pytest

new program endpoint

See merge request !52

feat: exclude internal_not, owner_ids and created & updated fields from show serializer for calendar

This reverts commit 072a351f.

This reverts commit d1681848.

This reverts commit 8d32fab1.

This reverts commit 906b8420.