title: Django filter many to many field in admin?
body: <p>I have three objects:</p>
  
  <pre><code>Thing
    some fields
  
  Bucket
    things = models.ManyToManyField(Thing)
  
  User
    buckets = models.ManyToManyField(Bucket)
    things = models.ManyToManyField(Thing)
  </code></pre>
  
  <p>When editing my "Bucket" field in the admin, I want to be able to select Things, but only those things that are in the "Things" list on the user currently logged in.</p>
  
  <p>How can I do that? In  other words, how can I filter the list of Things in the Bucket admin?</p>
  
  <p>Thank you very much for your help in advance.</p>