Add playlists

This adds

• Playlist and PlaylistEntry models,
• create_playlist, destroy_playlist and update_playlist custom permissions,
• PlaylistFilter, that filters the playlists using contains_file_ids or show_ids,
• APIPlaylistViewSet, that injects the request context into the serializer and checks permissions while deleting a playlist,
• PlaylistSerializer. A playlist that contains an entry without file_id or uri or contains multiple entries with null duration is rejected. Additionally, the playlist_id or default_playlist_id fields are now a PrimaryKeyRelatedField to the Playlist .
• Tests for (most) of the "happy path" API requests,