Highlander Changes

Created by: Shadowmech88

When someone is made into a highlander, their mind.special_role is set to "highlander" so you can now check whether someone is a highlander. When a highlander decapitates another highlander, they will now receive a full heal, accompanied by the same particle effect that soulstones now use. Highlanders are now permahulks, making them immune to stuns as well as incapable of using guns. Highlanders can no longer be buckled to vehicles. Upon use of the highlander button, all player-controlled silicons will be made into humans, so that they can then be made into highlanders. No more MoMMIs running around with claymores or engiborgs plasmaflooding while everyone else is fighting. Plasmamen and vox no longer lose the gear that they need to survive when they're turned into highlanders.

Closes #9801 (closed).