Included commits:
* xblock-external-ui: Include CSRF token in the API answer
* xblock-external-ui: Adds support for CORS headers (cross-domain request)
* xblock-external-ui: Include full path when building local_url
* xblock-external-ui: Fix TestHandleXBlockCallback & bok_choy, add tests
* xblock-external-ui: Only return `instance` in `_invoke_xblock_handler()`
* xblock-external-ui: Group resources by hash tag to avoid duplicate loads
* xblock-external-ui: Alternate referer check for CORS requests