Bug 22584: Add YAML support for Codemirror

Bug 22584: Add YAML support for Codemirror

This patch adds the dependencies required to provide YAML syntax
highlighting in CodeMirror text areas. The OpacHiddenItems system
preference configuration is updated to use a "text/x-yaml" CodeMirror
syntax.

To test, apply the patch and go to Administration -> System preferences
-> OPAC.

Expand the OpacHiddenItems preference and test that YAML-formatted text
entry has the correct syntax highlighting.

Signed-off-by: Michal Denar <black23@gmail.com>
https://bugs.koha-community.org/show_bug.cgi?id=2700

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>