R1:78c248d33024

Support drag-and-drop to set cover images on workboard cards

Summary: This was slightly more complex than I believed, but not too terrible.

Test Plan:
{F1096126}

- Also used some normal file uploaders to make sure I didn't break that.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15202