Refactor split migrator

LMS-2936

Removed a bunch of loc_mapper code
Cleaned up offering now being course and run in a few places
Removed extra db queries

Conflicts:
	common/lib/xmodule/xmodule/modulestore/tests/test_publish.py
yeah this seems odd, `user_id=int(identifier)` so this should be searching on `User.id` not `User.username`