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
Can we clarify this to be:
`args = "course_key email <new_org new_course new_run>"`

That is, the caller must specify all 3 (org, course, run) or none will be used.