Use In-Reply-To heades for email updates on issues

To facilitate threading and sorting of the email notifications sent from the issue tracker, use an In-Reply-To header. Whether the Message-ID of the first notification ("New issue...") or of the previous notification is used is probably not so important. Both choices would make it easier to deal with a larger number of issue notifications in the email client. I for one would prefer the first option, In-Reply-To the "New issue" mail.