uploads: autorefresh thumbnails on multi-file uploads page.

Automatically refresh thumbnails on the multi-file upload page as images
are processed in the background.

app/models/upload_media_asset.rb

@@ -29,6 +29,10 @@ class UploadMediaAsset < ApplicationRecord
     q.apply_default_order(params)
   end
 
+  def loading?
+    pending? || processing?
+  end
+
   def finished?
     active? || failed?
   end