Made husks crushable

Reasoning
* cool effect, feels right (idea stolen from Tiberium Wars)
* clean up the battlefield for better overview together with https://github.com/OpenRA/OpenRA/pull/4673
* can be used to deny captures: balance https://github.com/OpenRA/OpenRA/pull/4517
* fixes an exploit where people rush with 1tnk to block refinery or factory using it's husk http://youtu.be/ZzHp4sK0ryU?t=3m3s