[ticket/14717] Quote strings beginning with @ or % in yaml definitions

This type of use was deprecated in symfony 2.8 and will be dropped in 3.x.
Also see: https://github.com/symfony/symfony/pull/16285

PHPBB3-14717

phpBB/config/default/container/services_auth.yml

@@ -103,7 +103,7 @@ services:
     auth.provider.oauth.service.twitter:
         class: phpbb\auth\provider\oauth\service\twitter
         arguments:
-            - @config
-            - @request
+            - '@config'
+            - '@request'
         tags:
             - { name: auth.provider.oauth.service }