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
Does it use it the same way the old code did? We had what I think was an unresolvable discrepancy, where split always had a draft, but old-mongo didn't. Is that still/now an issue?