title: Add css class to all admin form field
body: <p>I need to assign a boostrap class to all my user's field in Django admin form, I wrote this code but it does not work.</p>
  
  <pre><code>  formfield_overrides = {
      models.CharField:     {'widget': TextInput(attrs={'class': 'form-control'})},
      models.CharField:     {'widget': EmailInput(attrs={'class': 'form-control'})},
      models.DateField:     {'widget': DateTimeInput(attrs={'type': 'date', 'class': 'form-control'})},
      models.EmailField:    {'widget': EmailInput(attrs={'class': 'form-control'})},
      models.BooleanField:  {'widget': CheckboxInput(attrs={'class': 'form-control'})},
   }
  </code></pre>
  
  <p>Can you help me?</p>