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 way I was thinking about the `branch_settings` is that it specifies the desirable access of the various branches (draft preferred, published only, ..).  Here, you need to know the NAME of the branch that should be used as master. 
By the way, shouldn't it be BranchName.published by default (instead of draft)?