R1:d94c33b61aa5

Use a more modern design for "no reviewers" warning in Differential

Summary: Ref T2222. Currently this is a giant header box thing. Move it into the ObjectBox.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8301