make csrf_token() usable without forms

The Twig function `csrf_token()` is currently only registered when the
Form component is installed. However, this function is also useful, for
example, when creating simple login forms for which you do not need the
full Form component.

Author: xabbuh

Resources/config/security_csrf.xml

@@ -21,5 +21,10 @@
             <argument type="service" id="request_stack" on-invalid="ignore" />
         </service>
         <service id="Symfony\Component\Security\Csrf\CsrfTokenManagerInterface" alias="security.csrf.token_manager" />
+
+        <service id="twig.extension.security_csrf" class="Symfony\Bridge\Twig\Extension\CsrfExtension">
+            <tag name="twig.extension" />
+            <argument type="service" id="security.csrf.token_manager" />
+        </service>
     </services>
 </container>