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
Since the new Publishing UI is now once again calling `compute_publish_state`, we need to figure out how to test for it.