Add Full image support to channels.
- ImageBackground enum has changed to ImageTheme, and the enum's values have changed as well
- TemporalField has been refactored to use generics
- Channel/ChannelGroup history elements now base their equality on start time only, to allow
de-duping of history events with the same timestamp on output.
Merge branch 'mbst-5686-ingest-pa-channel-logos' into stage