Be more selective about when to display buttons for joining a team

Specifically, before the first move, we never want any buttons,
(instead, the first player will commit to a team by making a move).

After the first move, there are two cases:

1. A player is already on a team:

	In this case we want only one button, labeled "Switch" to
	change to the other team.

2. A player is just spectating:

	This is the only case where we need to display two buttons,
	one labeled "Join X" and one labeled "Join O".