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