re-architecture: Coordinator.startScene(), StoryAssembler.continueScene() should be abstracted

[Logodaedalus]

We need to further separate (MVC-style) the module dependencies so that all the stuff specific to our displays is done independently of the data structures of Coordinator, etc.

Ideally, it should probably go like:

1. Coordinator.init()
2. do stuff with data properties now set in Coordinator for scene title screens
3. Coordinator.startScene(_Coordinator, playGameScenes[0], true); is called
4. Some kind of JS object is returned from that
5. kick off stuff with that, keying off StoryAssembler.handleChoiceSelection() ????

Use cases we need to handle:

1. Our two display templates retaining all functionality
2. It being as painless as possible to create new templates (piping in choices, etc)
3. It being as painless as possible to run StoryAssembler headless, to facilitate the testing harness, data viz, and any other shenanigans.