Stop setting a shortcode to newly-created media attachments (#16730)

* Stop setting a shortcode to newly-created media attachments

The WebUI has stopped using the “short media URL” in ages. This isn't used
anywhere except for mail notifications.

Deprecating it would allow us to eventually get rid of at least a database
column and corruption-prone index, as well as a controller.

* Fix tests

app/serializers/rest/media_attachment_serializer.rb

@@ -40,7 +40,7 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
   end
 
   def text_url
-    object.local? ? medium_url(object) : nil
+    object.local? && object.shortcode.present? ? medium_url(object) : nil
   end
 
   def meta