Ports /tg/'s lower latejoiner antag cap with slight modifications

Lower cap code by Ikarrus!
There's some modifications by me aswell.

This should help make rounds less boring, especially with lots of latejoiners. If the ideal crewmember:antag ratio gets too low, the next latejoiner is guaranteed to become an antag. Works in traitor, traitorchan and changeling rounds.

My modifications are listed in the commits after the first one.