empires 0.7: Add endpoints to add, remove, and list spectators

The endpoints are:

  POST /spectator
  DELETE /spectator/<ID>
  GET /spectators

The addition of a separate notion of spectators from players gives us
two advantages:

  1. People can just watch if they want to

  2. A client can naturally separate the selection of the player's name
     (to create the spectator, join the game, see who else is in, etc.),
     from the selection of the player's character. This separation should
     avoid some confusion we saw in playtesting where players confused
     which name was which.