Move Maniphest to modular mail commands

Summary:
Ref T7199. This fully modularizes mail command handling in Maniphest.

I had to add a couple of minor not-totally-solid-feeling tricks to deal with the "create" case, but they feel not-too-bad, and a million times better than what came before.

Test Plan: Used all commands with receive-test.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12240