uploads: change loading indicator for thumbnails.

Use a spinner icon instead of the word "Loading" for thumbnails that are
being processed in the background in a batch upload.

Also use morphdom to update thumbnails so we only update the parts of
the DOM that actually changed.

package.json

@@ -23,6 +23,7 @@
     "jquery-ui": "^1.13.0",
     "lodash": "^4.17.21",
     "mini-css-extract-plugin": "^2.5.1",
+    "morphdom": "^2.6.1",
     "postcss": "^8.4.5",
     "postcss-import": "^14.0.2",
     "postcss-loader": "^6.2.1",