Networking revisions

[mweidner037]

The networking code hasn't been looked at for a year or so and needs revising to match the project's current goals and style.

• Server implementations: use ES6 classes
• Make WebRTC a BroadcastNetwork instead of CausalBroadcastNetwork
• WebRTC: support multiple apps, > 2 users
• Move message splitting from MatrixWidgetNetwork to DefaultCausalBroadcastNetwork, with size configurable by its BroadcastNetwork
• Make sure BroadcastNetwork is as simple as possible, or conforms to existing interfaces (e.g. Observable?), so that users can easily integrate it with their own backends

[mweidner037]

I'll subsume this into "make more demos" in the roadmap.