[PR] Adds rare antag token drops for low population servers

[Torchwick]

I am excited to see this tried out. Abuses of this system can easily be ahelped and patterns could be recognized to lead to antag bans, etc. One concern that RubberRats mentioned in discord (that I agree is a worry of mine too) is that this may lead to people creating an ‘antag-only’ character on RP. I think that if this PR goes well, it would probably be beneficial to add a rule against this. The pattern of people that do this would be recognized quickly and a rule would just settle everything in stone.