Rough implementation of drag-and-drop file uploads

Summary:
This gets all the major pieces working. Allows you to drag-and-drop files in
Differential and Phriction, and embed files in remarkup with {Fxxx} references.
See also task.

I'm explicitly not documenting this yet since it's still pretty rough.

Test Plan: Dragged and dropped stuff into Differential and Phriction.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran, tomo
Commenters: tomo
CC: aran, tomo, jungejason
Differential Revision: 674