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