Empathy: Add routes to receive prompts and votes on prompts

The received votes are added to the prompt objects themselves, so we
don't need to send anything beyond the prompts, and the votes will
just ride along "for free". Also, because prompts are a part of the
Empathy game's "state" property, they will always be sent as part of
the "game-state" event sent when a new client joins, so we don't need
any explicit code in the Empathy class to take care of that either.