Move Differential to modular mail commands

Summary: Ref T7199. Convert Differential to modern modular commands.

Test Plan: Used bin/mail receive-test to send command and comment mail to Differential.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12239