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.
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
Provide workarounds for skip_draft where other code doesn't want its behavior
Support xml blocks w/ multiple parents
Use new check counter & bump count
Support import of ErrorDescriptors.
Use continue version for create course side effects
Disable fetching by version guid for now
More branch override precedence refinement
Code review changes re split import/export, create_xblock
Merge pull request #4633 from edx/dhm/create_xthing

Make import/export work with split mongo, replace create_xmodule with create_xblock