Apply Diffusion reply email directly with transaction editor

Summary: Ref T4896. Invoke the new editor directly instead of in a roundabout way when handling Audit email.

Test Plan: Used bin/mail receive-test to simulate mail, saw comment post with proper content source.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10127