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
The `ModuleStoreEnum.BranchName` enum was created specifically for `Split`.  Are you saying the `BranchName.published` and `BranchName.draft` enum-values are not sufficient?  We can definitely change their string-values so it is equivalent to what we want to persist in the database.