Implemented Deviator nerve gas missiles as warheads

This does the proposed refactoring from https://github.com/OpenRA/OpenRA/issues/2838 now that the pre-requisites https://github.com/OpenRA/OpenRA/pull/6030 are in place and makes the owner change temporary as in the original. It was permanent, because I lacked the coding skills at the time when I hacked AttackLoyality. 
Okay, in fact I had this on the warhead in the first commits. Not sure why I changed that.