Merge branch '3.4' into 4.1

* 3.4:
  [Debug] ignore underscore vs backslash namespaces in DebugClassLoader
  [TwigBridge][Form] Prevent multiple rendering of form collection prototypes
  [FrameworkBundle] fix describing routes with no controllers
  [DI] move RegisterServiceSubscribersPass before DecoratorServicePass
  Update ValidationListener.php
  [Yaml] ensures that the mb_internal_encoding is reset to its initial value
  [WebLink] Fixed documentation link
  [Security] getTargetPath of TargetPathTrait must return string or null
  [Hackday][Serializer] Deserialization ignores argument type hint from phpdoc for array in constructor argument
  [Security] defer log message in guard authenticator
  merge conflicts
  Fix HeaderBag::get phpdoc

Author: nicolas-grekas

Extension/Validator/EventListener/ValidationListener.php

@@ -51,7 +51,7 @@ public function validateForm(FormEvent $event)
         $form = $event->getForm();
 
         if ($form->isRoot()) {
-            // Validate the form in group "Default"
+            // Form groups are validated internally (FormValidator). Here we don't set groups as they are retrieved into the validator.
             foreach ($this->validator->validate($form) as $violation) {
                 // Allow the "invalid" constraint to be put onto
                 // non-synchronized forms