R1:f090fa7426e4

Use object PHIDs for "Thread-Topic" headers in mail

Summary:
Depends on D19009. Ref T13053. For "Must Encrypt" mail, we must currently strip the "Thread-Topic" header because it sometimes contains sensitive information about the object.

I don't actually know if this header is useful or anyting uses it. My understanding is that it's an Outlook/Exchange thing, but we also implement "Thread-Index" which I think is what Outlook/Exchange actually look at. This header may have done something before we…