Anki add-on: Proper image paste on 403

Sometimes when pasting image from browser, you get 403 error and the image won't download. Plain <img src="[external URL]"> is pasted instead. This leads to the image missing in the future, or card requiring internet connection to work. This addon fixes the issue. Original PR Anki is undergoing some redesign in editor code, so PR won't probably merge in its current state. Use this addon for now.