Add mobile upload button to Conpherence

Summary: Fixes T11622. Moves the remarkup upload button into the text area on mobile/tablet.

Test Plan: Mobile/Tablet/Desktop Conpherence

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11622

Differential Revision: https://secure.phabricator.com/D16626