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 messages.

Test Plan:
Sent a reply email locally, got a response with both Message-IDs in
"references".

Reviewed By: rm
Reviewers: gregprice, rm
Commenters: gregprice
CC: aran, gregprice, epriestley, rm
Differential Revision: 499