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
There is a bunch of code here that looks similar to the logic for `clone_course`.  Is there a reason that `SplitMigrator` is needed as a separate mechanism than `clone_course`?