R1:c0e26c65e03e

Make mail delivery reasons code-based; include positive and negative reasons

Summary:
Ref T7731. Looking forward to T5791, I eventually anticipate writing an interface which looks like a webmail UI where users can review mail they've been sent and understand why they recieved (or did not receive) the mail. Roughly like `bin/mail list-outbound` / `bin/mail show-outbound` work today, but policy-aware (so you can only see messages where delivery was attempted to you).

We currently record a list of "reasons"…