Replace create_xmodule with create_xblock

@cpennington @nasthagiri please review

* The primary task of this is to do all of the import/export; so, the title's wrong
* reconciles the signature of create_xblock
* uses make_course_key from cale's branch
* uses nimisha's asset fixes
* lots of import/export fixes
for easier code readability consider putting L206-209 in a sub-method with a docstring explaining that Split always keeps modules in Draft, while Mongo could have them in either Draft or Published.  Then L211-214 would also be replaced by the sub-method.