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!
Ah, I see now why it was needed, I readded it - without it, `SITE_NAME` points to 'localhost:80', which is incorrect for the bok-choy configuration, and makes this acid-xblock test fail: https://github.com/edx/acid-block/blob/e46f9cda8a03e121a00c7e347084d142d22ebfb7/acid/static/js/acid.js with canonical resource URLs https://github.com/edx/edx-platform/pull/2968#discussion-diff-19345396R124 .