Reverse-proxy friendly

run behind a reverse-proxy (with different base URL) without changes.

I.e. if a reverse-proxy server is proxying https://server.org/term/ to tty.js at http://localhost:8001/, tty.js should "just work".  Requires that URLs be relative in .html files and some minor cleverness in static/tty.js to determine the URL of the socket.io endpoint.   (Javascript approach borrowed from etherpad.)