[BE] Add/Modify comment section for edit content button for Freemarker

For the button, there's an HTML comment in the markup:

<!-- {"HST-Type":"MANAGE_CONTENT_LINK","uuid":"18e36c35-429d-4fee-b76e-eeabcbfc08bb"} -->

The idea is to add an attribute "HST-Offline-Document":"true" if the uuid is referencing a document that is offline, i.e. has no live variant, meaning does not have hippo:availability=live.

For Freemaker based site, the class involved is org.hippoecm.hst.tag.HstManageContentTag.
Its #processHippoBean already contains the handleNode and cmsUser of which the the JCR session should have has access to the document variants.