title: Unable to render content of custom page from custom controller in active_admin
body: <p>There are custom page like:</p>
  
  <pre><code># app/admin/csv/bar_csv.rb
  ActiveAdmin.register_page 'bar_csv' do
    content do
       ...
    end
  end
  </code></pre>
  
  <p>and custom controller like:</p>
  
  <pre><code># app/admin/bar/bar.rb
  ActiveAdmin.register Bar do
    controller do
      def index
        ...
        # I expect render content of bar_csv
        render 'admin/bar_csv'
      end
    end
  end
  </code></pre>
  
  <p>I expect to render content of <code>bar_csv</code> as custom controller when I access <code>admin/bars</code>. But I encountered <code>Template is missing</code>.
  I tried <code>render 'admin/bar_csv/index'</code>, <code>render 'bar_csv/index'</code>, <code>render 'admin/bar_csv'</code> and <code>render 'bar_csv'</code> as well</p>
  
  <p>Any help would be appreciable.</p>