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
This may be better implemented using the new `default_store` contextmanager.  So rather than getting direct access to the underlying via the internal method, you can set the `default_store` to `split`.  The `source_modulestore` would automatically be accessed via Mixed's APIs using the original course_key.