Add handling for a spectators list in addition to the players list

The model is that when people initially join the game, (providing a
name for themselves), they are a spectator. They then move from the
spectator list to the player list when then they choose a character
name.

The server isn't very clever about this. It has distinct endpoints for
adding/removing players and adding/removing spectators. So in this
code we have to explicitly remove ourselves as a spectator when we add
ourselves as a player.