[#6328] ticket:448 Set In-Reply-To to parent's Message-ID
[#6328] ticket:448 Set Message-ID for first ticket notification

When ticket is created set Message-ID: in corresponding notification to
ticket.message_id().  Thus later we can use this value in top-level
comments notifications as In-Reply-To: and References: to group emails
in one thread.
[#6328] Test cleanup

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>