Delete Asset Manager assets when corresponding attachment is deleted

This wires up an instance of ServiceListeners::AttachmentDeleter so that
the Asset Manager assets corresponding to an AttachmentData are deleted
via a Sidekiq worker.

This will mark the relevant assets as deleted in Asset Manager. While
there is functionality in the Asset Manager API to restore a deleted
asset, there doesn't appear to be any such functionality in Whitehall.

Note that the Asset Manager assets will only be deleted if no other
non-deleted Attachment instances are still referencing the
AttachmentData, i.e. where AttachmentData#deleted? is `true`.

Also note that it looks as if when a (consultation) Response or a
PolicyGroup is destroyed, the associated attachments are not deleted.
While it's not obvious that it's possible to delete a Response via the
user interface, it does appear to be possible to delete a PolicyGroup.

While it doesn't feel appropriate to address this here, I'd recommend
that for consistency and completeness, both of these problems should be
fixed by calling Attachable#delete_all_attachments from a before/after
destroy callback on Response and PolicyGroup. This would mean a
'destroy' event would be published to the attachment notifier for each
attachment and the relevant assets would be deleted in Asset Manager.

config/initializers/attachment_notifier.rb

@@ -6,4 +6,9 @@
         .update!
     end
   end
+  notifier.subscribe('destroy') do |_event, attachment|
+    ServiceListeners::AttachmentDeleter
+      .new(attachment.attachment_data)
+      .delete!
+  end
 end

test/integration/attachment_deletion_integration_test.rb

@@ -44,6 +44,11 @@ class AttachmentDeletionIntegrationTest < ActionDispatch::IntegrationTest
         get @attachment_url
         assert_response :not_found
       end
+
+      it 'deletes corresponding asset(s) in Asset Manager' do
+        Services.asset_manager.expects(:delete_asset).with(asset_id)
+        AssetManagerAttachmentDeleteWorker.drain
+      end
     end
 
     context 'when draft document is discarded' do
@@ -58,6 +63,11 @@ class AttachmentDeletionIntegrationTest < ActionDispatch::IntegrationTest
         get @attachment_url
         assert_response :not_found
       end
+
+      it 'deletes corresponding asset(s) in Asset Manager' do
+        Services.asset_manager.expects(:delete_asset).with(asset_id)
+        AssetManagerAttachmentDeleteWorker.drain
+      end
     end
   end