Spectator mode

[tomwwright]

It would be valuable to watch a game be played out, either in review or in real time

Desirable features:

• join a game as a spectator
• ability to resolve turns in the same way as when playing
• live updates on when a new turn happens
• ability to set "active" player to view game from their POV

[tomwwright]

Refactor and abstract the client mode -- GameStore and UiStore both seem to contain logic for specific "playing" behaviour