title: Django admin: Second level ordering in list_display
body: <p>While showing a model in Django's admin's list_display, I'd like to sort first by one column, then by another.<br>
  Here's an example model:</p>
  
  <pre><code>class Customer(models.Model):
      name  = models.CharField(max_length=30)
      the_date = models.DateField()
  </code></pre>
  
  <p>How could this be achieved?</p>