Change local media attachments to perform heavy processing asynchronously (#13210)

Fix #9106

db/migrate/20200306035625_add_processing_to_media_attachments.rb

@@ -0,0 +1,5 @@
+class AddProcessingToMediaAttachments < ActiveRecord::Migration[5.2]
+  def change
+    add_column :media_attachments, :processing, :integer
+  end
+end