style: if ! changed by unless

- if ! changed by unless as it is more readable
- !object.blank? is equivalent to object.present?, also more readable

app/controllers/admin/questions_controller.rb

@@ -67,7 +67,7 @@ module Admin
     def destroy
       if can? :destroy, @question
         # Do not delete global questions
-        if !@question.global
+        unless @question.global
 
           # Delete question and its answers
           begin