Provide some diagnostic tools for examining inbound and outbound mail

Summary: We can't show this stuff on the web UI because it has password reset links and private reply-to addresses, but we can provide easier CLI tools than "root around in the database". Land a rough version of `bin/mail show-inbound` and `bin/mail show-outbound`.

Test Plan: Used both commands to examine mail from the CLI.

Reviewers: btrahan

Reviewed By: btrahan

CC: tido, euresti, aran

Differential Revision: https://secure.phabricator.com/D5963