This endpoint removes the need setting media eintries through the media
endpoint. This is helpful because a lot of operations that clients will
usually do, like adding new, deleting old, or reordering media sources
becomes non-atomic when operating on lists.

The /program/calender endpoint returns an envelope object that returns
related objects for the query.

Without the fix drf-spectacular would advertise the response type as an
array of CalendarSchema even though it is just the CalenderSchema.

The update logic of the repetition_of field did not consider cases in
which the episode field of TimeSlot objects is deleted.

This fix populates the TimeSlot’s repetition_of_id field with a subquery
instead of using a separate database field for it.

The decision to keep repetition_of as a separate database field was made
on the assumption that this would be a performance problem. However, the
subquery for the repetition_of field is quite performant, especially
with a separate database index. In contrast, the update logic for a
materialized repetition_of field is quite complex and it is easy to miss
cases that should have updated the field.

As long as we don’t have an actual use case in which retrieving
timeslots becomes slow because of this subquery, we should refrain from
materializing the field.

This change makes the notes independent of timeslots and renames them to
episodes.

Multiple timeslots can now refer to the same episode introducing a new
and more convenient model for repetitions that does not duplicate
content.

This change also makes it possible for users to pre-plan their episodes
without the need for radio planners to create a timeslot for the episode
first.

refs #249
refs aura#382

This reverts commit 09853c06.