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.

app/views/upload_media_assets/_preview.html.erb

@@ -5,7 +5,7 @@
     <% if upload_media_asset.failed? %>
       Failed
     <% else %>
-      Loading
+      <%= spinner_icon(class: "animate-spin h-8") %>
     <% end %>
   <% end %>