Fix parsing of "Reviewed By" field

Summary: In the Message parser, we read this field and expect to get an array of PHIDs out of it. Currently, we get a string. Instead, get an array of PHIDs.

Test Plan: Wrote a message like "Fixes Tnnn" with "Reviewed by: duck", and saw no more parse error during message parsing.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8510