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
Why is `import_xblock` defined here instead of `ModuleStoreWrite`?  Is there anything about `import_xblock` that assumes only 2 branch types (draft and published)?  According the to the API, it doesn't seem to.