R1:e80c59cbc622

Introduce basic `bin/mail` with a `resend` workflow

Summary:
Fixes T2458. Ref T2843. @tido's email from T2843 has exhausted its retries and failed, but we want to try it again with the patch from D5464 to capture the actual error. This sort of thing has come up a few times in debugging, too.

Also fixed some stuff that came up while debugging this.

Test Plan:
- Ran command with no args.
- Ran resend with no args.
- Ran resend with bad IDs.
- Ran resend with already-queued messages, got "already…