Unconditionally enable drag-and-drop uploads for Remarkup text areas

Summary: Make these always work. Notably, this makes them work in Maniphest. Previously this was at odds with stuff fixed in D3651.

Test Plan: Dragged and dropped files into Remarkup in Maniphest.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3652