Mailer on the CC list of Maniphest Task.
Summary: External user gets on the CC list of a task.
Test Plan: {F42630}
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Maniphest Tasks: T2804
Differential Revision: https://secure.phabricator.com/D5853