Issue 293 part two

More work on Issue #293.

This PR creates the single_version symlink at the end of the build, and modifies the Nginx config to serve the single version docs.

So far, this only works for subdomain URLs, not for CNAMEs.

I tried to modify the Nginx config as little as possible to reduce the chance of site breakage, but I sure would like to figure out a good way to do some real testing of it before pushing it to the live site.