Support drag-and-dropping web URLs of images

[hypest]

Drag-and-drop of images recently landed on develop in WordPress-Android (see https://github.com/wordpress-mobile/WordPress-Android/pull/4397) but with the intentional omission of not supporting URLs to images. The rationale of the omission was that it is inefficient to verify that the URL does point to an image.

It would be nice to add that functionality though and so, following @aforcier 's suggestion, I'm opening this ticket so we can revisit.