Drag-and-drop upload for Maniphest

Summary:
This needs a bunch of UI polish (critically, it's totally undiscoverable) but it
basically works correctly. I'll clean it up in some followups.

Test Plan:
Uploaded some files via drag-and-drop, made comments, etc.

Reviewed By: aran
Reviewers: tomo, aran, jungejason, tuomaspelkonen
CC: anjali, aran
Differential Revision: 332