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/views/notification_mailer/_status.html.haml

@@ -37,7 +37,7 @@
                                   %p
                                     - status.media_attachments.each do |a|
                                       - if status.local?
-                                        = link_to medium_url(a), medium_url(a)
+                                        = link_to full_asset_url(a.file.url(:original)), full_asset_url(a.file.url(:original))
                                       - else
                                         = link_to a.remote_url, a.remote_url