Bug 985596 Set up initial desktop conversation window

This gets a basic conversation window up and running - when a push notification is received, the conversation window will be opened and automatically start the call (controls to come later). This makes it possible for the complete round-trip call to be set up.

This uses a lot of code from the shared directories, so the main glue here is the html and the ConversationRouter.

Client.js has moved out to the shared directory and is in loop-client already.

I'm not intending that we land LoopService.js without major rework, but I cleaned it up a bit anyway so that it didn't have unnecessary comments & debug.