title: How to pass a variable to form_theme?
body: <p>I want to theme my form so that the field's label show the current locale, something like </p>
  
  <blockquote>
    <p>Name (en) :</p>
  </blockquote>
  
  <p>So I would like to rewrite block generic_label like that :</p>
  
  <pre><code>{# form_theme.html.twig #}
  
  {% block generic_label %}
  {% spaceless %}
      {% if required %}
          {% set attr = attr|merge({'class': attr.class|default('') ~ ' required'}) %}
      {% endif %}
      &lt;label{% for attrname,attrvalue in attr %} {{attrname}}="{{attrvalue}}"{% endfor %}&gt;{{ label|trans }} (app.session.locale)&lt;/label&gt;
  {% endspaceless %}
  {% endblock %}
  </code></pre>
  
  <p>and import it in my template :</p>
  
  <pre><code>{% form_theme options 'myBundle:Object:form_theme.html.twig' %}
  </code></pre>
  
  <p>but the app variable is not accessible in the form template.
  How can I pass a variable to a form theme ?</p>