RCStream: use recommended Nginx config for WebSockets

Use the configuration recommended by
<https://github.com/miguelgrinberg/Flask-SocketIO/blob/9ed9b070cccd98be88ce3b3de0f684e4601924db/docs/index.rst>.

Change-Id: Ie4056af6ad45f902f3a4365bca3cc5dbea46a0f4