Merge branch '2.8' into 3.0

* 2.8: (25 commits)
  [Console] Fix an autocompletion question helper issue with non-sequentially indexed choices
  [Process] Fix pipes handling
  Mentioned the deprecation of deep parameters in UPGRADE files
  [Filesystem] Fix false positive in ->remove()
  [Filesystem] Cleanup/sync with 2.3
  [Validator] Fix the locale validator so it treats a locale alias as a valid locale
  [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
  [2.8] update readme files for new components
  fix lowest TwigBridge deps versions
  reference form type by name on Symfony 2.7
  [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
  ...

Conflicts:
	UPGRADE-2.8.md
	UPGRADE-3.0.md
	src/Symfony/Bridge/Twig/composer.json
	src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Compiler/FragmentRendererPass.php
	src/Symfony/Component/Console/Helper/DialogHelper.php
	src/Symfony/Component/Console/Helper/ProgressHelper.php
	src/Symfony/Component/Console/Tests/Helper/LegacyDialogHelperTest.php
	src/Symfony/Component/HttpFoundation/composer.json
	src/Symfony/Component/Security/Core/README.md
	src/Symfony/Component/Security/Csrf/README.md
	src/Symfony/Component/Security/Http/README.md
	src/Symfony/Component/Validator/Constraints/LocaleValidator.php

Author: nicolas-grekas

README.md

@@ -18,10 +18,7 @@ The documentation for the component can be found [online] [0].
 Resources
 ---------
 
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Component/Ldap/
-    $ composer install
-    $ phpunit
-
-[0]: https://symfony.com/doc/2.8/components/ldap.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)