Switch the MessageId macro to using the msgid-search path

The msgid-search path defaults to listing all the mailing lists that the
mail reached, does the right thing if the mail was sent to only one list
but also allows appending /firsthit to just get directed to the mail.
The current path used just gets the mail and can't show all the lists.

By default we probably want readers to be able to see all discussions
that were started by a single Message-ID but some message links will
be appropriate to be directed directly to the mail itself. So the
msgid-search path is the most appropriate link to use.

The Debian mailing lists also use the msgid-search path in the
List-Archive mail message header.