title: flash[:alert] not working, but flash[:notice] displays the message on redirect
body: <p>I am using rails_admin gem and the configuration is as follows </p>
  
  <pre><code>config.authorize_with do
    if current_user.nil? || current_user.role != 'admin'
      redirect_to main_app.root_path
      flash[:alert] = "Sorry you are not authorized!"
    end
  end
  </code></pre>
  
  <p>When I use <code>flash[:notice]</code>, I can see the message on the root_path, but if I change it to <code>flash[:alert]</code> it does not display, any ideas why and what would be the solution?</p>
  
  <p>I want to use <code>:alert</code> only, since changing the notice color will result it displaying red font for all other notices.</p>