Ask to mute a request's update emails

The page an update email's mute link opens: it names the request and its organization and asks, and changes nothing, since mail security scanners fetch every link in a message. Its button posts the link's d and t to POST /hd/unsubscribe. A link that names no request, or one of a desk that does not take customers now, opens a page that names none. Needs no credential; each client address spends a budget of reads a minute.