Update to FOSUserBundle 3.1

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

src/Wallabag/ApiBundle/DependencyInjection/Configuration.php

@@ -17,9 +17,6 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('wallabag_api');
-
-        return $treeBuilder;
+        return new TreeBuilder('wallabag_api');
     }
 }