For some reason the episode result set was unstable when using the
has_timeslots filter. Additionally, the former solution always added a
join to the query, even if the the has_timeslots filter wasn’t used.

fix: add missing source parameter

See merge request !64

Without `source="media"` a POST request to the "episodes" endpoint fails when `mediaId` is set.

This didn't come up far, because Dashboard only uses updates and Django does some magic when setting model attributes on existing instances.

This is helpful for applications like the dashboard that use static
typing and can use the `AnyPermission` type to catch typos and
missing/renamed permission names during the build phase.