title: Activeadmin custom sortable column in index
body: <p>I have custom sortable column 'Status' in index, how can I make it sortable?  </p>
  
  <pre><code> index do
      selectable_column
  
      column :title
      column :counter
      column 'Status', do |purchase|
        if purchase.status == 'verified' &amp;&amp; purchase.expiration_date &lt; DateTime.now.utc
          'ended'
        elsif purchase.status == 'verified' &amp;&amp; purchase.expiration_date &gt;= DateTime.now.utc
          'active'
        else
          purchase.status
        end
      end
    end
  </code></pre>