Don't end warmup when teams are unbalanced (WRT sv_teamnagger)

Don't end warmup when teams are unbalanced (WRT sv_teamnagger)

Allows spectators to see the unbalanced teams warning, because they
could fix it by joining the smaller team.

Aborts countdown if teams become unbalanced, for the case of someone
joining this can be avoided by enabling g_balance_teams_queue.

Updates the chatcon notification for countdown abort when the player
count drops below the minimum.