title: Django admin filter urls for not-equals
body: <p>I've created a custom <code>FieldListFilter</code> for my site's admin for whether <code>modeltranslation</code> translations are available for a field.</p>
  
  <p>It creates a filter that currently looks like this on the sidebar:</p>
  
  <p><img src="https://i.stack.imgur.com/8sQHo.png" width="132" height="92"></p>
  
  <p>The "Lacks English" option adds <code>vt_title_en__exact=</code> to the query string.</p>
  
  <p>I would like to also implement a "Has English" which does the opposite filter. Can I do that with a django admin site url query strings?</p>
  
  <p>Btw: source for my custom filter: <a href="https://github.com/yairchu/azlemi/blob/master/vote/admin.py#L30" rel="nofollow noreferrer">https://github.com/yairchu/azlemi/blob/master/vote/admin.py#L30</a></p>