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
Also, `create_course` in split allows the `root_block_id` to be overriden with a value other than `course`.  Will this code have the expected behavior in that case?