Fix form required_css_class not applied for individual fields

The `required_css_classes` are missing from the labels
```html
<div class="row">
  <div class="col-sm-4">
    {% for field in form.visible_fields|slice:":-2" %}
      {{ field|bootstrap }}
    {% endfor %}
  </div>
  <div class="col-sm-8">
    {% for field in form.visible_fields|slice:"-2:" %}
      {{ field|bootstrap }}
    {% endfor %}
  </div>
</div>
```

While it did work for
```
{{form|bootstrap}}
```