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
