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/components/media_asset_preview_component/media_asset_preview_component.html.erb

@@ -30,7 +30,7 @@
     <% else %>
       <div class="media-asset-placeholder rounded border border-solid flex items-center justify-center w-<%= size %>px h-<%= size %>px">
         <% if media_asset.processing? %>
-          Loading
+          <%= spinner_icon(class: "animate-spin h-8") %>
         <% elsif media_asset.failed? %>
           Failed
         <% else %>