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
b/c @nasthagiri said I should explain that these default to the source course's values. *Here* (not "there") they merely default to None. nvm, I'll flip the docstrings.