Depends on D19620. Ref T13077. This adds a "Publish" operation which points the current version at some historical version of the document -- not necessarily the most recent version. Newer versions become "drafts".

This is still quite rough and missing a lot of hinting in the UI, I'm just making it work so I can start making the UI understand it.

Used the "Publish" action to publish older versions of a document, saw the document revert. Many UI hints are missing and this operation is puzzling and not yet usable for normal users.