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
cloning does the publish more efficiently via copying the published head pointer to the draft head. import should perhaps also do that, but b/c import is written to be modulestore agnostic it doesn't have that awareness.