xblock-external-ui: Add XBlock API call to render XBlock views

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
Merge pull request #6 from edx-solutions/xblock-external-ui-solutions

Xblock external UI - Server-side integration