Destroy Objective Probability scales with population

Instead of being a flat 1% or 2% it will now use prob(100/ticker.minds.len) which will give it roughly the same chance as any crewmember would have of being targetted for assasination.
A little more fair for traitor objectives
Apparently New Players have minds so that's not going to be appropriate any more.
Merge pull request #4692 from Ikarrus/destroyscale

Destroy Objective Probability scales with population