R1:349686319e30

Allow the published version of a Phriction document to differ from the most recent version

Summary:
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.

Test Plan: Used the "Publish" action to publish older…