title: Django - CheckboxSelectMultiple() with scrollbar
body: <p>I will have more than a hundred checkboxes on the form. This looks ugly right now. I am trying to make the checkboxes be in a frame(?) with a vertical scrollbar. Using <code>height:100px;</code> in crispy forms doesn't have any affect at all. Using that same in <code>attr</code>to <code>CheckboxSelectMultiple()</code> or <code>ModelMultipleChoiceField</code> throws <code>unexpected argument attr</code> error.</p>
  
  <p>Form field:</p>
  
  <pre><code>specialities = forms.ModelMultipleChoiceField( queryset=spQry ,widget = forms.widgets.CheckboxSelectMultiple(),  label=_('Speciality'), required=True)
  </code></pre>
  
  <p>cripsy form part:</p>
  
  <pre><code> self.helper = FormHelper()
          self.helper.layout = Layout(
              Field('practice_date'),
              Field('gender'),
              Field('specialities', style='max-height:20px!important'),
              StrictButton("Submit", name="submit", value="Submit", type='submit' ,css_class='btn btn-action-success')
          )
  </code></pre>
  
  <p>Is there anyway to have that scrollbar?</p>
  
  <p>Thank you</p>