title: Bootstrap 3 not working in Symfony3
body: <p>I've just started to learn Symfony3, and I'm trying to theme my form using bootstrap3. According to the <a href="http://symfony.com/doc/current/cookbook/form/form_customization.html" rel="nofollow noreferrer">documentation</a>, <code>bootstrap_3_layout.html.twig</code> is built-in and can be activated globally by altering the Twig settings in <code>config.yml</code>:</p>
  
  <pre><code># Twig Configuration
  twig:
      debug:            "%kernel.debug%"
      strict_variables: "%kernel.debug%"
      form_themes: 
          - "bootstrap_3_layout.html.twig"
  </code></pre>
  
  <p>When I reload the page, the page loads without CSS, the twig file and page source are as follows:</p>
  
  <pre><code>{# app/Resources/views/experiment/new.html.twig #}
  
  {{ form_start(form) }}
  {{ form_widget(form) }}
  {{ form_end(form) }}
  
  
  &lt;form name="form" method="post"&gt;
  &lt;div id="form"&gt;&lt;div class="form-group"&gt;&lt;label class="control-label required" for="form_title"&gt;Title&lt;/label&gt;&lt;input type="text" id="form_title" name="form[title]" required="required" class="form-control" /&gt;&lt;/div&gt;&lt;div class="form-group"&gt;&lt;button type="submit" id="form_save" name="form[save]" class="btn-default btn"&gt;Create Task&lt;/button&gt;&lt;/div&gt;&lt;input type="hidden" id="form__token" name="form[_token]" class="form-control" value="O7tN2AUjyLByEz2tkNf675jqixxjev0p4ivzs5Bn4VA" /&gt;&lt;/div&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Which shows the bootstrap3 classes have been injected correctly. The site itself looks like this:</p>
  
  <p><a href="https://i.stack.imgur.com/7W5Wi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7W5Wi.png" alt="enter image description here"></a></p>
  
  <p>What step(s) have I missed to get the CSS to work correctly? </p>