viewer / remote: support multiple concurrent sessions

[noerw]

For multi-seat usage in the context of using the IVE platform for remote / from-home studies, we need to separate the websocket events into session namespaces.

• socket.io has a feature just for that, this should be easy to implement
• server-side stored state in controllers/sockets.js also needs to be namespaced by session-ID
• Session IDs could be randomly generated could be passed via an URL parameter to the viewer and remote.
• For the remote, a further enhancement would be to
  • listing active sessions, so they can be easily switched without manually changing the URL.
  • creating new sessions