Support reverse web proxying with websocket upgrade

[oberstet]

We support reverse proxying of Web traffic using the reverseproxy webservice - but that doesn't work currently for reverse proxying jupyterhub, probably because HTTP requests with upgrade to websocket isn't forwarded correctly.

Enable using the following config as part of a web transport, to reverse proxy the URL /jupyterhub to a locally running jupyterhub:

"jupyterhub": {
    "type": "reverseproxy",
    "host": "127.0.0.1",
    "port": 8000,
    "path": "/",
    "options": {
        "enable_websocket": true
    }
}

For general setup of reverse proxy with jupyterhub, see https://jupyterhub.readthedocs.io/en/stable/reference/config-proxy.html. A general overview on the technical architecture of jupyterhub is here: https://jupyterhub.readthedocs.io/en/stable/reference/technical-overview.html