add an interface for export which are lists

Lists does not supports aggregators.

The export manager will filter the aggregators applying on the export
depending on the interface implemented by the export: if the export implements
`Export\ListInterface`, no aggregators will be returned for this export.

Resources/views/Form/fields.html.twig

@@ -58,6 +58,7 @@
             {{ form_widget(child) }}
             {{ form_label(child, '', {'label_attr': { 'class' : 'inline'} }) }}
         </span>
+        <br/>
     {% endfor %}
     </div>
     <script type="text/javascript">chill.checkNullValuesInChoices("{{ form.vars.full_name }}");</script>