#1405144 by chia: make sure we don't add multiple event listeners for 'message' if there is a socket reconnection.

#1942356: Fixed bug that caused the 'client-message' process to be emitted twice on server.js for each message received on a socket

fix getBackendUrl(), and actually call it from sendMessageToBackend(), thanks amitaibu for the prompting

first cut at contentChannel notifications functionality. per-channel, configurable disconnect and connect notifications

refactor returning the list of connected users to a content channel, expose it via http for the backend.