Refactor split migrator

LMS-2936

Also, a bunch of ancillary cleanups.
Removed a bunch of loc_mapper code
Cleaned up offering now being course and run in a few places
Removed extra db queries

Replaces https://github.com/edx/edx-platform/pull/4357
When the draft and published content are the same, doesn't studio still have UI changes to indicate that the content has an active draft? If so, won't this code screw that up?