Update to FOSUserBundle 3.1

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

src/Wallabag/CoreBundle/DependencyInjection/Configuration.php

@@ -9,8 +9,8 @@ class Configuration implements ConfigurationInterface
 {
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('wallabag_core');
+        $treeBuilder = new TreeBuilder('wallabag_core');
+        $rootNode = $treeBuilder->getRootNode();
 
         $rootNode
             ->children()