title: Django: how to create custom "base" model
body: <p>In almost all my tables (= classes of <code>models.Model</code>) I have three <code>DateTimeField</code>:</p>
  
  <ul>
  <li>creation</li>
  <li>validity start</li>
  <li>validity end</li>
  </ul>
  
  <p>Is there a way to have a "base" model class where I declare those fields, and make all my other model extend this one? I couldn't find a valuable answer on the Web.</p>