No longer hide unseen content as part of Getty ingest

- i.e. don't set activelyPublished=false
- no need to iterate; no need for a ContentLister
Allow starting Getty job from any arbitrary offset
Permit stopping the Getty ingest
Merge pull request #587 from atlasapi/getty-ingest-fixes

Alter Getty ingest task to work on partial list

- Allow arbitrary start offset for content query to be specified via SchedulingStore
- No longer hide unseen content as part of Getty ingest