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
OK - the comment is just worded weirdly. "TODO: remove this import - xmodule should not depend on django" would be much clearer.