Add Message-Id header to emails

The previous code, which was submitting locally, apparently didn't
need to provide a Message-Id header.  However, now that we're
directly submitting to a remote system, we need to make sure that
a Message-Id header exists or the emails will get bounced.

In addition, the Python docs for this module state that Message-Id
is really one of the required fields anyway.  It's unclear how many
real bugs we lost because of this, but I got involved when there
was complaining on IRC that a bug submitted didn't show up on the
-bugs list.

In case folks are wondering why I'm committing/pushing this (or how),
I've already fixed this on wrigleys (thanks to Andrew Gierth for
helping me debug and test the changes) and subsequently gave myself
access to this repo, to get this commit in, before anyone else
commits and overwrites my local hacks and breaks the bugs form again.