Merge branch '2.8' into 3.4

* 2.8:
  [HttpFoundation] update phpdoc of FlashBagInterface::add()
  bug #27701 [SecurityBundle] Dont throw if "security.http_utils" is not found (nicolas-grekas)
  [Validator] Fix the namespace of RegexTest

Author: nicolas-grekas

Tests/Constraints/RegexTest.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Constraints;
+namespace Symfony\Component\Validator\Tests\Constraints;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Validator\Constraints\Regex;