Allow tablet breakpoint in Conpherence to have persistent participant pane

Summary: tablet viewpoints here are wide enough to support the participant column, go ahead and give it dedicated space.

Test Plan: review tablet, desktop, and mobile breakpoints in conpherence with column toggled on and off

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16724