Implement heartbeat caching

[eshapiro42]

The various data sent by a game's heartbeat to the frontend can be cached so that events are only emitted when relevant data is updated.

• [x] Refactor display_* methods in Interaction, Turn, etc. so all data is gathered by the Heartbeat object and caching is handled centrally.
• [x] Add a "refresh" button to the frontend to clear the hearbeat cache and force all events to be re-emitted.
• [x] Similarly, clear the heartbeat cache when a player rejoins a game.
• [x] Update the readme.