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.
Merge pull request #516 from atlasapi/bt-channels

Ingest BT channel metadata
Save channel groups in channels, and cache groups in API
Merge pull request #518 from atlasapi/bt-channels

Save channel groups in channels, and cache groups in API
Add placeholder property for BT channels
Dummy audience stats data
Update alias namespaces for BT channels
Output protection should use that field
Add dummy ratings output
Ensure start and end dates are propagated through to the simple model ChannelNumberings
Temp fix that merges existing channel numberings on a channel with the newly ingested ones.
This is not perfect, but should solve the existing issue.
Merge pull request #519 from atlasapi/mbst-9187-channel-numbering-date-range

start/end dates on channel numberings
Merge pull request #520 from atlasapi/fix-channelgroup-link-removal

Fix ChannelGroup link removal
Add updaters to create "all channel" groups
Merge pull request #521 from atlasapi/bt-all-channels

Add updaters to create "all channel" groups
One-off ingest of audience data WIP
Replace dummy audience data with real simplification
Add AudienceStatisticsModelSimplifier
Merge pull request #522 from atlasapi/mbst-9152-audience-data

One-off ingest of audience data
Avoid division by zero when calculating mean