title: django admin action without selecting objects
body: <p>Is it possible to create a custom admin action for the django admin that doesn't require selecting some objects to run it on?</p>
  
  <p>If you try to run an action without selecting objects, you get the message:</p>
  
  <pre><code>Items must be selected in order to perform actions on them. No items have been changed.
  </code></pre>
  
  <p>Is there a way to override this behaviour and let the action run anyway?</p>