Fully modularize mail commands

Summary: Ref T7199. Everyone can have a mail command! You can have a mail command! You can have a mail command! Mail commands for everyone!

Test Plan: Used bin/mail receive-test to issue commands against files and pastes.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12238