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" why a mail is undeliverable, but this list is string-based (so it can not be translated once we start persisting it) and has only negative reasons (so it can not be used to fully understand reasons for delivery or nondelivery).

Make it code-based (so it can be translated) and allow both positive and negative reasons to be listed (so positive reasons can be understood).

Test Plan: Used bin/mail show-outbound to review mail delivery reasons, including the positive reason we currently have (forced delivery of authentication mail).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7731

Differential Revision: https://secure.phabricator.com/D12297