R1:c915a064a940

Include both the Gmail and "natural" Message-IDs in the "References" header

Summary:
See T251, where gregprice correctly argues that we need both:

None of the other people on the thread will have seen that message, so it
seems
like a lot of clients would put the server's message in a new thread. In
general, I think you want the References: header to mention every ancestor
message in the thread that you know about, because that's how MUAs keep a
thread
together in the face of missing some of its…