Adding download buttons to invoice (orders#show)

config/locales/en.yml

@@ -6,5 +6,10 @@ en:
   broken_file: Warning! this file is broken
   delete_file_cofirmation: Are you sure you want to delete the file %{filename}?
   digital_delivery: Digital Delivery
+  digital_download: Download %{type} ↓
+  digital_format:
+    mp3: Audio MP3
+    mobi: Kindle eBook
+    epub: ePub eBook
   spree_digital:
     upload: Upload