Fix height of send message area on Conpherence
Summary: When I tweaked the mobile layout, this didn't get updated. Reduces height of send message area.
Test Plan: test layout on mobile and desktop
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9237