Provide Console YAML export tool (as ‘content’)

Users (most often developers) should be able to export a YAML + binary file representation of an arbitrary JCR node from the CMS console, with output delivered as a zip file. This should produce output that is easy to add to a maven project for re-import via the new bootstrapping process, similar to how XML exports were used previously.

SNS usage needs to be verified to work properly as well. Note that the an SNS node itself is not allowed to be exported (as content root path).