Adds a Syndicate encryption key to uplinks

Partially inspired both by #1700 and a post in the traitor items idea thread this simple addition allows traitors to eavesdrop on the radio channels of every department on the station as well as talk confidentially with any other traitor that also has the key.

Unlike #1700 there's use to this even if nobody else picks it and it's unlikely that security will be able to realize a traitor has it unless the traitor tells them. Considering how in normal gameplay traitors and nukeops never coexist I don't see any real potential for abuse there and it also shouldn't be bad for double agents either as it could prompt some interesting situations.