Handle disconnects & reconnects properly

[yuvipanda]

If the websocket disconnects now, the end user has to do a refresh of the page.

Once #77, #78, #79 get merged, we can detect these disconnects and try to reconnect.

[consideRatio]

I think the initial connection attempt maybe benefit from this also. I've seen the initial connection fail regularly when starting up locally, forcing a page reload - if it fails or not on the first attempt may be related to ping, and the fact that jupyter-server-proxy will finalize the handshake between client/proxy before proxy/server is even initialized (jupyterhub/jupyter-server-proxy#459) which makes initial traffic possibly disrupted.