Fix EZP-21531: As a user I need a way to pick correct Content name language

* Added a ContentHelper in `eZ\Publish\Core\Repository\Helper`, so that translated names can be easily retrieved from PHP (added Twig helper that uses it).
* Replaced usage of some deprecated Twig classes
* Moved translated field extraction logic to ContentHelper

And of course, everything is unit tested ! :bowtie: