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

@cpennington @nedbat @ormsbee Can you have a quick look to check that I'm on the right path with this? It seem to work well, rendering either individual blocks or verticals containing both XBlocks and XModules. If that's the correct approach, I'll flesh it out more, add tests and start working on the client side, but I could use a sanity check : ) Thanks!
Sure - `local_resource_url()` is being used by XBlocks to get a URLs to their resources. Using an absolute path without including the domain works as long as the XBlock is being displayed on the same domain - but when the XBlock is displayed on another domain, it makes it attempt to load its resources from that other domain instead of querying the LMS.