[Editor] Add loading spinner for image drag-and-drop upload #12443

Describe the bug

Today, when someone in the editor (/new) drags-and-drops an image to upload it, they have no indication that it is currently uploading because there is no spinning indicator. This UX bug sets incorrect expectations on how our product works.

To Reproduce

  1. Go to /new
  2. Drag an image file into body area to upload an image
  3. Wait for image to upload

This is how it incorrectly works here:

current image upload

Expected behavior

2021-01-26_Editor-Upload-Image-Fix

The easiest solution here would be to render the same "Loading..." indicator when people upload an image from the "Upload image" button.

Desktop (please complete the following information):

  • OS, version: Mac OS 11
  • Browser, version: Firefox