Tighten up message pane in Conpherence

Summary: Tweaks a number of things in the Conpherence message pane.

Test Plan: Review a Conpherence in Chrome/iOS

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6007