Don't show participants in Conpherence left open on mobile

Summary: Fixes T11764. Moves rendering of the column to client-side, which can skip if it detects we're on mobile.

Test Plan: Open column on desktop, switch to mobile, don't see column. Toggle column on mobile on and off. Switch back to desktop.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11764

Differential Revision: https://secure.phabricator.com/D16725