Change when a share link stops working

Set a working link's expiry, counted from now rather than added to what is left, or send expires_in_days: null to take its end date away so it works until somebody turns it off. Only a link that still works can be changed: a revoked or already-expired one is refused, because its URL has been out of use and handing that same URL back is not an edit to a date. Share the file again with a new link instead. The file's LEVEL is never a refusal on its own: a date is not an audience, and a link on a private file is that file's address like any other. Prolonging a publication is publishing, so this needs the same authority as deciding who may read the file: the person who added it or a workspace admin. The one exception is a file already at 'Anyone with the link', whose audience is already decided and whose links any editor may keep working. That authority is answered before the link is looked up: 403 when you may read the file but may not decide its audience, whatever link id you name, and otherwise 404 when the file or the link does not exist, has been deleted, or belongs to another workspace.