Differential - special-case "no reviewers" warning to show only for revions that need review

Summary: 'cuz who cares unless you need review?

Test Plan: noted the UI showed up appropriately to my new business logix

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2010

Differential Revision: https://secure.phabricator.com/D3958