title: Django max_length for IntegerField
body: <p>I have this model field:</p>
  
  <pre><code>id_student = models.PositiveIntegerField(primary_key=True, max_length=10)
  </code></pre>
  
  <p>The <code>max_length</code> restriction doesn't work. I can log in to the admin and create a student with an id with more than 10 chars. How can I solve this?</p>