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
Can we clarify this by saying:

'If the user didn't specify an org OR course OR run, the missing values will be taken from the original course.'