Merge branch '2.7' into 2.8

* 2.7:
  [HttpFoundation] Fix transient test
  [HttpFoundation] Add a dependency on the mbstring polyfill
  [2.7] update readme files for new components
  add readme files where missing
  fix lowest TwigBridge deps versions
  [EventDispatcher] fix syntax error
  Don't use reflections when possible
  Don't use reflections when possible
  [Form] Update form tests after the ICU data update
  [Intl] Update tests and the number formatter to match behaviour of the intl extension
  [Intl] Update the ICU data to version 55
  [Intl] Fix the update-data.php script in preparation for ICU 5.5
  [Process] Fix memory issue when using large input streams
  Use constant instead of function call.
  fixed test name
  automatically generate safe fallback filename
  [Console] default to stderr in the console helpers

Conflicts:
	composer.json
	src/Symfony/Bridge/PhpUnit/README.md
	src/Symfony/Bridge/Twig/composer.json
	src/Symfony/Component/Console/Helper/DialogHelper.php
	src/Symfony/Component/Debug/DebugClassLoader.php
	src/Symfony/Component/HttpFoundation/composer.json
	src/Symfony/Component/Security/Acl/README.md
	src/Symfony/Component/Security/Core/README.md
	src/Symfony/Component/Security/Csrf/README.md
	src/Symfony/Component/Security/Http/README.md

Author: nicolas-grekas

README.md

@@ -9,15 +9,8 @@ the Java Spring framework.
 Resources
 ---------
 
-Documentation:
-
-https://symfony.com/doc/2.8/book/security.html
-
-Tests
------
-
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Component/Security/Http/
-    $ composer.phar install --dev
-    $ phpunit
+  * [Documentation](https://symfony.com/doc/current/components/security/index.html)
+  * [Contributing](https://symfony.com/doc/current/contributing/index.html)
+  * [Report issues](https://github.com/symfony/symfony/issues) and
+    [send Pull Requests](https://github.com/symfony/symfony/pulls)
+    in the [main Symfony repository](https://github.com/symfony/symfony)