Export configuration YAML strings as multiline

Problem/Motivation

Some configuration values are multiline (mails, system.maintenance & etc) but their export contains "\r\n" to split lines.
That makes configuration merging hard for such strings and mostly impossible to use configuration translations management for big config objects (for example webforms #2836206: Translation of big webforms broken → )

Proposed resolution

Beautify export and make diff/merge tools less painful
http://symfony.com/doc/3.1/components/yaml.html#dumping-multi-line-liter...

Remaining tasks

agree on approach
patch, commit

User interface changes

No

API changes

TBD

Data model changes

All config files that have multiline strings will be updated on next export

Release notes snippet

Configuration files with multi-line strings will be exported using Symfony's multi-line literal block formatting option, improving readability and diffs. This will mean a configuration diff the first time a configuration file is re-exported.