Clients Should Be Able To Subscribe to Socket Messages

[Olliebrown]

While the socket.io message system needs to be able to send any events that the EDSDK might have as messages, a connected client should only receive messages it has explicitly opted in to. This can be achieved by having 'subscription' routes as part of the normal REST API.

Thoughts on implementation:

• Socket.io already has a mechanism for grouping clients and servers into rooms that we can use to implement this.
• There should be a route for subscribing to a message of some type and a route for unsubscribing
• One subscription route with a path parameter for the message type should suffice (and a similar unsubscribe route)