Allow CCs to be specified and edited for mocks

Summary: Not sure if we're going to do reviewers or whatever, but let mock creators share mocks with lists or individuals.

Test Plan: Created a mock with "duck" and "dog" cc'd, verified they showed up on the email.

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2568

Differential Revision: https://secure.phabricator.com/D5056