title: Adding extra class apart from span5, sonata admin bundle
body: <p>i have this </p>
  
  <pre><code>&lt;div class="form-group controls sonata-ba-field sonata-ba-field-{{ sonata_admin.edit }}-{{ sonata_admin.inline }} {% if errors|length &gt; 0 %}sonata-ba-field-error{% endif %} {% if not has_label %}sonata-collection-row-without-label{% endif %}"&gt;
  
      {{ form_widget(form) }}
  
      {% if errors|length &gt; 0 %}
          &lt;div class="help-inline sonata-ba-field-error-messages"&gt;
              {{ form_errors(form) }}
          &lt;/div&gt;
      {% endif %}
  
      {% if sonata_admin.field_description.help %}
          &lt;span class="help-block sonata-ba-field-help"&gt;{{ sonata_admin.admin.trans(sonata_admin.field_description.help, {}, sonata_admin.field_description.translationDomain)|raw }}&lt;/span&gt;
      {% endif %}
  &lt;/div&gt;
  </code></pre>
  
  <p>This form_widget creates the fields on a sonata edit page, however by default it has the <code>class="span5"</code></p>
  
  <p>I would like to edit it so it also has </p>
  
  <pre><code>form-control
  </code></pre>
  
  <p>i tried to modify the twig by setting the attributes manually but then the extra classes that are defined in admin controllers don't get applied.</p>