Show upload status, success and failures for drag-and-drop files in notifications

Summary: Currently, we do a poor job of communicating drag-and-drop upload errors. Show progress and success/failure in notifications.

Test Plan:
{F20671}

{F20672}

Uploaded files to maniphest attachments, remarkup text areas, Files tool.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3655