Display the page title on the WAYG

Previously the page_title block was rendered, but that was not set. The
application_name was set with the 'gateway.second_factor.choose_second_factor.title'
and using that on the page title made perfect sense to me.

src/Surfnet/StepupGateway/GatewayBundle/Resources/views/second_factor/choose_second_factor.html.twig

@@ -5,7 +5,7 @@
 {% block container_class %}{{ parent() }} wayg{% endblock container_class %}
 
 {% block content %}
-    <h2>{{ block('page_title') }}</h2>
+    <h2>{{ block('application_name') }}</h2>
     {{ form_start(form, {'attr': {'class': 'form-horizontal choose-second-factor-type'}}) }}
 
     {% set formErrors = form.vars.errors.form.getErrors(true) %}