Enforce joining the zombie team

[Lartza]

An enhancement idea, which I am far too unskilled to work on (don't really know how pyspades works :) even though I know Python)

Not sure if this can be done due to pyspades mechanics, but if the teams are even and a new player connects, he should be only able to join the zombie team. Current situation is that on the quiet hours human team is always bigger, and winning.

This together with autobalance should help the issue.