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
Nothing in xmodule should depend nor use django. here's one of the prime culprits. There's no time to fix this now, but it violates our arch (and has for a long time).