Load primary reviewer PHID

Summary: A cursory look at DifferentialReviewer suggests the primary reviewer doesn't actually have to be among the reviewers? Uploading this so bug reporter can patch and see if it helps.

Test Plan: Nope.

Reviewers: epriestley

Reviewed By: epriestley

CC: szymon, aran, Korvin

Maniphest Tasks: T1625

Differential Revision: https://secure.phabricator.com/D3198