Improve drag-and-drop uploader

Summary:
Make it discoverable, show uploading progress, show file thumbnails, allow you
to remove files, make it a generic form component.

Test Plan:
Uploaded ducks

Reviewed By: tomo
Reviewers: aran, tomo, jungejason, tuomaspelkonen
CC: anjali, aran, epriestley, tomo
Differential Revision: 334