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. 
We will want to update this for any other type of ownership change (scripts, hijackers/engineers in other mods).  Something like the following should work:

```csharp
bool changingOwner;

public void Start(Actor self, Player newOwner)
{
	remaining = info.Duration;

	changingOwner = true;
	self.ChangeOwner(newOwner);
	changingOwner = false;
}

public void OnOwnerChanged(Actor self, Player oldOwner, Player newOwner)
{
	if (!changingOwner)
		originalOwner = newOwner;
}
```