Make contracts its own antag type

[Firebarrage]

Id agree thats it seems a bit rare, but I don't think it needs to be its own mixed type antag. Ideally antags have at least some amount of freedom in how they play e.g. allowing for some level of stealth, but this locks you into something pretty specific.

Alternatively - do we know if these show up in surplus crates? Because id be absolutely OK with these being in surplus crates.