title: Hide ActiveAdmin menu but does not disable it
body: <p>In my AA app, I use the following instruction to hide the menu I do not want non admin users to see:  </p>
  
  <pre><code>menu :if =&gt; proc{ current_user.is_admin? }
  </code></pre>
  
  <p>but this does not prevent those users to enter /admin/users in the url and to access the users menu content. What is the best way to prevent non admin user to access hidden menu ?</p>