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 some minor changes:

• Show mail bodies.
• Show more mail information.
• Start renaming "MetaMTA" to "Mail", at least in the web UI.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5791, T7013, T9141

Differential Revision: https://secure.phabricator.com/D13878