title: django admin enable sorting for calculated fields
body: <p>I have the following two fields in my db table and model (Model Name: Order):</p>
  
  <pre><code>id, branch_id, product_id, cost, quantity, status, ordered_at
  </code></pre>
  
  <p>And I have the following code in my OrderModelAdmin:</p>
  
  <pre><code>list_display = (
      'order_number',
      'branch',
      'product',
      'cost',
      'quantity',
      'calculated_total',
      'status',
      'ordered_at',
  )
  
  def calculated_total(self, obj):
      return obj.cost * obj.quantity
  calculated_total.short_description = _('Total')
  </code></pre>
  
  <p>Now, I want to enable sorting for this field. In reality, all I need to do is to add a column in my SELECT statement:</p>
  
  <pre><code> SELECT (t.cost * t.quantity) as TOTAL
   ORDER BY TOTAL
  </code></pre>
  
  <p>Is there a way I can append an SQL statement for sorting in Django Admin?</p>