Use TigerVNC + unix sockets

[ryanlovett]

See TigerVNC/tigervnc #639. This should simplify nbnovnc.

[manics]

I've got unix sockets working as a Dockerfile using the upstream Websockify and TigerVNC releases: https://gist.github.com/manics/7d1f4b76ce06c2bb07db88e3496a1561

In conjunction with jupyterhub/jupyter-server-proxy#151 and using the wrapping support in websockify (runs vncserver as a child process so no need for another supervisor) it might be possible to simply this extension to just managing the configuration.