Allow disabling the ability to roll sleeper agent.

[JOELED]

If you turn on/off the traitor preference in the save slot you're playing mid-round it changes whether or not you're available for sleeper rolls. I use this for when I don't feel like rolling for traitor but wouldn't mind sleeper, or vice versa -- The fact it's already an option if you remember to flip the toggle after you've already joined the round (and know that that's something that works to begin with) does make me feel like it would feel better to just have them be distinct options