game/tictactoe: Expand player to include a team property

game/tictactoe: Expand player to include a team property

The existing /player endpoint is expanded to allow a player to choose
which team to join. The team must be either an empty string, (to leave
all teams and be just a spectator), or must be one of the defined
teams of the current game. As expected, TicTacToe defines its teams as
"X" and "O".

Commit includes testing.