title: Django model, using auth_group as a ForeignKey
body: <p>I would like to use the <code>auth_group</code> table as a foreign key in my custom model :</p>
  
  <pre><code>class MyModel(models.Model):
      field = models.ForeignKey(auth_group)
  </code></pre>
  
  <p>I cant find a mention on how to reference this table. </p>
  
  <p>I've seen <code>settings.AUTH_USER_MODEL</code>, but couldnt see anything for <code>auth_group</code> </p>
  
  <p>Any pointers would be appreciated.</p>