Open a version in a preview session

Mints a short-lived URL that opens one page of this version — including a private one — with its stylesheets, scripts and images authorized too. The URL is meant to be framed: following it once exchanges the proof it carries for a cookie on the version's own origin and redirects to the same page without it, so the framed document holds no secret. The session authorizes that version only. Requires the same permission as minting a file link.