title: What is "swappable" in model meta for?
body: <p>Looking tough django auth models code, I came across this bit of code:</p>
  
  <pre><code>class User(AbstractUser):
      class Meta(AbstractUser.Meta):
          swappable = 'AUTH_USER_MODEL'
  </code></pre>
  
  <p>It's obvious that it has something to do with the new <code>AUTH_USER_MODEL</code> setting in settings.py, but how does it actually work, by what python "trick"?</p>
  
  <p>And in what other situations can it be used?</p>