Allow drags from other browser windows.

Which is unnecessarily complicated.

Test Plan:
* open https://ka-perseus-graphie.s3.amazonaws.com/bde3297f79638611aa830a582e906e49b1d5cf46.png
* drag it into a perseus one question area
* see the image inserted into the area
* drag an image from my desktop
* see the same (well almost... /assessment_items/images 500'ed, but it
  would have been the same otherwise)

Auditors: alex