title: Override default queryset in Django admin
body: <p>One of my models has a deleted flag, which is used to hide objects globally:</p>
  
  <pre><code>class NondeletedManager(models.Manager):
      """Returns only objects which haven't been deleted"""
  
      def get_query_set(self):
          return super(NondeletedManager, self).get_query_set().exclude(deleted=True)
  
  class Conversation(BaseModel):
      ...
      deleted = models.BooleanField(default=False)
      objects = NondeletedManager()
      all_conversations = models.Manager() # includes deleted conversations
  </code></pre>
  
  <p>How can I override the default queryset used by Django admin module to include deleted conversations?</p>