[TASK] Escape reserved characters in yaml files

In preparation to update the symfony/yaml dependency to v4,
proper escaping needs to take place.
Following
http://symfony.com/doc/current/components/yaml/yaml_format.html#strings
unsufficient escaping will result in a parse error.

Change-Id: Ic6eb3c2825c8c877d292aaa21e4b0e1a9fee6fe4
Resolves: #84420
Releases: master
Reviewed-on: https://review.typo3.org/56292
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>

typo3/sysext/core/Documentation/Changelog/master/Important-84420-ProperlyEscapeReservedCharsInYaml.rst

@@ -0,0 +1,22 @@
+.. include:: ../../Includes.txt
+
+==========================================================
+Important: #84420 - Properly escape reserved chars in yaml
+==========================================================
+
+See :issue:`84420`
+
+Description
+===========
+
+If dealing with YAML files in the TYPO3 system - for instance to configure forms
+using the `form` extension or if configuring `ckeditor` - integrators should properly
+quote strings containing special characters like `@` or `%` to be upwards compatible
+with the version 4 symfony yaml parser.
+
+More information can be found in the Symfony_ docs.
+
+.. _Symfony:: http://symfony.com/doc/current/components/yaml/yaml_format.html#strings
+
+
+.. index:: Backend, Frontend, ext:form, ext:rte_ckeditor

typo3/sysext/core/Tests/Unit/Configuration/Loader/YamlFileLoaderTest.php

@@ -106,7 +106,7 @@ public function loadWithPlacholders()
 options:
     - option1
     - option2
-betterthanbefore: %firstset.myinitialversion%
+betterthanbefore: \'%firstset.myinitialversion%\'
 ';
 
         $expected = [