code: def getTransitionActor(obj, action_id):
      
      review_history = getReviewHistory(obj)
      for event in review_history:
          if event.get('action') == action_id:
              return event.get('actor')
      return None
docs: Returns the actor that performed a given transition. If transition has
      not been perormed, or current user has no privileges, returns None
      :return: the username of the user that performed the transition passed-in
      :type: string