refactor: use django_filters FilterSet for APIShowViewSet

This change re-implements all existing collection filters for the
APIShowViewSet with a FilterSet. No breaking changes are expected,
though there are some changes in semantics:

* The owner, host, musicfocus, language, category, and topic
  filters now accept multiple values (i.e. ?category=2,3)
* The owner, host, musicfocus, language, category, topic, and type
  filter values are now validated and may be rejected as invalid if
  the referenced object does not exist.