Allow disabling the ability to roll sleeper agent.

[junori]

If I'm playing a character that has traitor enabled I always get a little nervous when the sleeper roll comes around. And not in the good way. I totally believe that they could be separate, and I also agree that rolling sleeper agent can be a bit heavy at times. Especially when you *know* they just executed another antag and its like 60 minutes into a round of RP... yeesh. I personally don't mind rolling sleeper as much as I used to with the addition of dead drops, but I still would like to see them be separated.