Editor: handle drag and drop uploads

Updated the editor to handle drag and drop uploads. This required making
the entire editor footer a drag and drop zone. The required event
listeners have been added to handle dragEnter, dragLeave, and drop. The
drop listener pulls out the file list from the drop event and calls the
upload function with an array of images.
Editor: handle images only

Updated the editor to handle only image uploads. The input[type=file]
was updated with an additional attribute 'accept="image/*"' and will
only accept images types now. The drag and drop implementation checks
the file type of each incoming file and only uploads files that are of
an image type.