Ingest genres from PA onto channels. Add filters to both channels and channel_groups endpoints
to filter by channel_genre.
Tweaks to genre ingest:
 - update genre uri prefix to http://pressassociation.com/genres/
 - ensure genres are written on existing channels
 - correct invalid null check when resolving channels by alias
Actually finish the channel_groups filter
Copy ChannelGroup before filtering numberings by genre, because immutability is nice.
Merge pull request #557 from atlasapi/mbst-9629-channel-genres

Mbst 9629 channel genres