Refactor split migrator

LMS-2936

Also, a bunch of ancillary cleanups.
Removed a bunch of loc_mapper code
Cleaned up offering now being course and run in a few places
Removed extra db queries

Replaces https://github.com/edx/edx-platform/pull/4357
We need to reconcile this code with the current compute_publish_state implementation.  We have to have compute_publish_state return the same value for both module stores - in order to support both module stores in the current Studio.  Otherwise, we cannot land Split until the new UI is implemented.

I strongly recommend removing this method and instead fixing the `compute_publish_state` implementation in split_draft.