Conpherence - fix no messages button

Summary: make it work with z-index and make it grey.

Test Plan:
clicked the grey button and it worked! Safari and Chrome

clicked around and observed loading mask functioning correctly

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3396

Differential Revision: https://secure.phabricator.com/D6207