Respect unified view in Diffusion

Summary: Ref T2009. Respect preference and make 1up/2up options work properly.

Test Plan: Toggled 1up vs 2up in Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12015