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
My conception is that eventually we'll use split in such a way that published branch won't exist until the user decides to actually publish something. Thus, it shouldn't exist at the time of course creation