Clear all channel numberings known to be linking to PA channel groups on the resolved channels, to avoid
duplicate numberings if the PA rewrite history in their files.
Change channel group cache to be cache of group publishers. Also minor naming tweaks.
Merge pull request #528 from atlasapi/duplicate-numbering-fix

Clear PA channel numberings during PA channel ingest
Filter non-PA numberings from existing numbering set before merging in new PA numberings.
Merge pull request #529 from atlasapi/duplicate-numbering-fix-mk2

Filter non-PA numberings from existing numbering set before merging in new PA numberings.
Merge pull request #525 from atlasapi/bbg-giw-adaptors

Bloomber, GlobalImage, Getty adapters
Change filtering publisher from PA to METABROADCAST for channel numbering filtering code.
Merge TopicRefs each time we get a clip from Getty

This is slightly special because we know Getty never returns TopicRefs with offsets.

It should eventually be generalized into ContentMerger.
Merge pull request #530 from atlasapi/getty-merger

Merge TopicRefs each time we get a clip from Getty
Set image uri on described

The old image uri field is still required for content merging to
select the image, if the image is on a non-precident item.
Merge pull request #532 from atlasapi/bt-vod-set-legacy-image-field

Set image uri on described
Merge images block
Fix incorrect import of Strings
Merge pull request #531 from atlasapi/ios-deep-link-locations

Create deep links for iOS devices
Allow null images block
Correct BT Portal URI
Correct series URI construction
Allow for null service/player ids
Add new content groups

Creating for TV box sets, buy-to-own and Box Office.
Only allow one writer of channels at a time