Update to FOSUserBundle 3.1

Also remove some deprecation from Symfony.
Use `LegacyEventDispatcherProxy` to handle Symfony 4 dispatch from FOSUser

src/Wallabag/CoreBundle/ParamConverter/UsernameFeedTokenConverter.php

@@ -2,7 +2,7 @@
 
 namespace Wallabag\CoreBundle\ParamConverter;
 
-use Doctrine\Common\Persistence\ManagerRegistry;
+use Doctrine\Persistence\ManagerRegistry;
 use Sensio\Bundle\FrameworkExtraBundle\Configuration\ParamConverter;
 use Sensio\Bundle\FrameworkExtraBundle\Request\ParamConverter\ParamConverterInterface;
 use Symfony\Component\HttpFoundation\Request;