Adding support for dragging and dropping images into the markdown editor. Instead of having upload their image first and then link to that image, users can simply drag-and-drop their image file into the markdown editor. This will upload the image to reviewboard and insert a link to that image in the editor.
To accomplish this, a new resource for uesr file attachments was added (see https://reviews.reviewboard.org/r/6220/ for initial work) and some modifications to the markdown editor JS were made to support drag-and-drop (see https://reviews.reviewboard.org/r/5954/ for initial work).
Still a work-in-progress.

Ignore all the JavaScript files for now please