title: add a custom button 'download all files' in sonata admin bundle symfony2
body: <p>I want to have a button same as 'Add New' available in sonata bundle. I don't know how to add it.</p>
  
  <p>I found the code(base_list.html.twig) which creates 'Add New' but I don't know how making it customize for every page.</p>
  
  <pre><code>{% block actions %}
  &lt;div class="sonata-actions btn-group"&gt;
      {% include 'SonataAdminBundle:Core:create_button.html.twig' %}
  &lt;/div&gt;
  {% endblock %}
  </code></pre>
  
  <p>Any help will be really appreciated.</p>