www: /$INBOX/$MSGID/d/ to diff reused Message-IDs

www: /$INBOX/$MSGID/d/ to diff reused Message-IDs

To ensure users aren't abusing the ability to reuse Message-IDs,
provide a convenient front-end to `lei mail-diff' from WWW.
Most of the time it's just list-appended signatures, so I expect
this to be useful for /all/ users.