LMS-11179 Fix Assets with Split
Replace create_xmodule with create_xblock
Refactor all callers (often replacing with more appropriate fn)

Also, added for_branch_setting method to old mongo and removed some
branch verification tests

Conflicts:
	common/lib/xmodule/xmodule/modulestore/mongo/draft.py
	common/lib/xmodule/xmodule/modulestore/split_mongo/split_draft.py
	common/lib/xmodule/xmodule/modulestore/xml_importer.py
Refactor xml_importer.py for easier reading.
Remove post-publish step.
Revert import code removal for remapping private verticals.
Add make_course_key to modulestores, so that external clients don't need to assume a CourseKey subtype.

Conflicts:
	common/lib/xmodule/xmodule/modulestore/xml_importer.py
Trying to get split to work
Import depth first to help w/ publishing
fixup - re-enable Asset checking.
Move about overview creation to common superclass