Ingest BT channel metadata

Extra information is stored as channel groups, retrieved
from BT's MPX system.

Configuration remains to be done, once we have the various
environments from them.
Potentially. Have to run through channelgroup persistence logic to make sure there's no code that breaks without a type. While we're there, is ChannelNumbering the correct abstraction for the link between channel groups and channels for an arbitrary collection of channels?