title: Django make field disabled depending on another field value
body: <p>I'm new to django and trying to do something with an issue.I have this model:</p>
  
  <pre><code>class MyModel(models.Model):
      value = models.CharField(max_length=50, choises=...)
      custom_value = models.CharField(max_length=50, blank=True)
  </code></pre>
  
  <p>And I need custom_value field to be enabled to edit only if value of value field is "CUSTOM".</p>