Use TransactionEditor in Differential mail handling

Summary: Ref T2222. Moves this instance of CommentEditor to TransactionEditor.

Test Plan: Used `bin/mail receive-test` to test receiving comment mail and action mail.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8427