title: How to limit access to active admin to admin users
body: <p>I want that only my users who have their attribute <code>is_admin</code> set to true to be able to access my active admin backend</p>
  
  <p><em>how should I do this?</em></p>
  
  <p>"Normal" users should only be able to login to the site, not to active admin.</p>