Fix: Reorder assignments

This pull request gives staffs the permission to access the [show](https://github.com/professor/whiteboard/blob/master/app/controllers/assignments_controller.rb#L118) action and the [reposition](https://github.com/professor/whiteboard/blob/master/app/controllers/assignments_controller.rb#L130) action in assignment_controller, so as to fix the reposition functionality.