[3.4] Improved the overriding of templates

[Haehnchen]

{# templates/bundles/FOSUserBundle/layout.html.twig #}
{% extends '@FOSUser/layout.html.twig' %}
{# ... this doesn't work ... #}

{# templates/bundles/FOSUserBundle/layout.html.twig #}
{% extends '@!FOSUser/layout.html.twig' %}
{# ... this works as expected ... #}

https://symfony.com/blog/new-in-symfony-3-4-improved-the-overriding-of-templates

symfony/symfony#24179
symfony/symfony#24264
symfony/symfony#24064