title: Django Forms : disable field if booleanfield is checked
body: <p>Here is the definition of my form:</p>
  
  <pre><code>class WorkflowForm(forms.Form):   
      new_user = forms.BooleanField(required=False)
      user = ModelChoiceField(queryset=User.objects.all())
      description = forms.CharField(required=False, widget=forms.Textarea)
  </code></pre>
  
  <p>I would like to disable the <em>user</em> ModelChoiceField if the <em>new_user</em> field is checked.</p>
  
  <p>So is it possible to define in this class a dependency for a field to another, like in my case a modelchoicefield depend of a booleanfield checked or not?</p>