Symfony 4 update

[josemi-ca]

Just released this week along with PHP 7.2: https://symfony.com/blog/hello-symfony-4

[ParisLiakos]

We need to check whether the bundles we are using provide support for Symfony 4. Then we probably need to find a new home for it, i dont think that keeping it under siwapp/siwapp-sf3 makes sense anymore