R1:8c06d89070b9

Flesh out web UI for mail a bit to prepare for Herald outbound rules

Summary:
Ref T9141. Ref T5791. Ref T7013. Major changes here is:

- Currently, we don't store the headers we actually sent, or the reasons we actually did or did not deliver a mail.
- Start storing these (as `headers.sent` and `actors.sent`).
- Show them in the web UI.
- Show them in `bin/mail show-outbound` (previously, we sort of re-computed them in a hacky way).
- Take them into account in `bin/mail volume`.

Then…