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
@dmitchell Have @cpennington and team implemented this code in the `map_into_course` method in the OpaqueKey repo?