The HTTP-server is indeed custom. It has a server-side template system but it’s currently not in use. The JavaScript-front-end does HTTP RPC calls getting JSON back, so all the UI stuff is done in the browser (see Single-page application - Wikipedia ). Nowadays one would use Angular/React/Backbone etc. for that.
I send you a summary on how to build the templates.js via PM – please take a loko at that. This you have to do using the git checkout.