This change implements the frontend for letting users drag-and-drop images into

their markdown editors. This will upload the image as a user file attachment

and insert the relevant markdown syntax into the editor.
This is based on work by David Kus at https://reviews.reviewboard.org/r/6510/

Compared to that change, I've reworked the UI a bit to avoid the overlapping

drop targets (the review request file attachments drop target is now isolated

to the "Files" section, which appears momentarily if it wasn't already

visible). I've also done a bunch of refactoring of the code to modernize and

clean it up.
At the moment, this adds overlays over the entire text field, and will insert

the markdown text as a new line wherever the cursor is. I'd like to improve

this in the future to make it more obvious where the new entry will end up

(basically moving the cursor during the dragover with inline feedback) but

that's a larger change.

Dragged images into a variety of text fields.
Dragged files to create new file attachments.
Ran unit tests.
Ran js-tests.