Symfony 5 support

[lushc]

Integration with the templating component was dropped in Symfony 5.0, removing support for PHP templates and form themes. As a result the tests no longer pass because the class TemplatingRendererEngine doesn't exist.

I'm guessing that the symfony/templating dependency and Ivory\FormExtraBundle\Templating\FormExtraHelper should be removed entirely in favour of only supporting Twig templates, bumping the other Symfony components to ^5.0.