Changes Assassinate to use special_role if needed

Created by: Kammerjunk

Fixes #12233 (closed) Currently, assasinate/cult sacrifice objectives go solely off of target.assigned_role and sacrifice_target.assigned_role respectively which, if special roles like ERT members or wizards are in play, will make the objective read "Sacrifice/Assassinate Big McLargehuge, the MODE." This changes it to use the target's special_role (which seems to be designed specifically for this) if the assigned_role is MODE by ripping off someone else's work in code\datums\mind.dm I'm at a loss regarding how to test the cult part of it, so if anyone can assist there, that'd be super. I can't seem to select the game mode datum using SDQL (I guess you can't select datums?) and there's no link to it in the cultists' minds.