Replace deprecated Symfony classes and features

When E_USER_DEPRECATED error reporting level is enabled, it generates many messages about use of deprecated code in phpBB:

1. Templating code uses deprecated Twig classes.
2. http_kernel service does not give a fourth argument and the default behaviour in this case changes in future versions of Symfony.
3. Console exception_subscriber listens for a deprecated event.
4. Event dispatcher extends a deprecated class.
5. Private services are retrieved directly from the DI container (i.e. $container->get('foo')).

The attached patch should resolve these issues.