article editor drag-and-drop details

[eplebel]

IMAGES:

• DnDing single or multiple image files anywhere on AE will change color of key figure upload area to a very light green, communicating to user that DnDing is activated/will work (& thus that they won't lose any unsaved information) (for multiple-image icon, can use MD icon "photo_library")
  (the "3 + Move" is OS-specific, and so for this part, it doesn't really matter what it looks like as long as it works)
• once images are dropped (within upload area or anywhere on the page), images are uploaded, compressed, thumbnailed in the order of the file names (and appended to any pre-existing figures already attached to article). NOTE, that ideally a loading spinner should be displayed for each of the images as they're uploading/being processed (so if 4 images uploaded, 4 loading spinners should be displayed)