title: Sort Active Admin index by named scope
body: <p>I have a model <code>Slider</code> with a named scope: </p>
  
  <pre><code>scope :positioned, order("CASE WHEN position = 0 THEN 0 ELSE 1 END DESC").order("position ASC").order("created_at DESC")
  </code></pre>
  
  <p>This adds a somewhat complex ordering. I'd like my ActiveAdmin to re-use that scope. 
  So far, I can only order by a column, like so: </p>
  
  <pre><code>ActiveAdmin.register Slider do
    config.sort_order = "position_asc"
  end
  </code></pre>
  
  <p>How can I import, re-use or force the ordering from the named scope in the active-admin index?</p>