Delete underlying file/asset when parent edition is discarded

This rectifies the inconsistency in behaviour explained in the previous
commits.

Calling `Attachment#destroy` for each attachment on the edition
triggers the `after_destroy` callback on `FileAttachment` which in turn
will delete its `AttachmentData` and its underlying file and Asset
Manager asset if no other attachments are still referring to the
`AttachmentData`.

app/models/attachable.rb

@@ -124,7 +124,7 @@ def next_ordering
   end
 
   def delete_all_attachments
-    attachments.each { |attachment| attachment.update(deleted: true) }
+    attachments.each(&:destroy)
   end
 
   def reorder_attachments(ordered_attachment_ids)

test/integration/attachment_deletion_integration_test.rb

@@ -64,8 +64,15 @@ class AttachmentDeletionIntegrationTest < ActionDispatch::IntegrationTest
       it 'responds with 404 Not Found for attachment URL' do
         logout
 
-        get @attachment_url
-        assert_response :not_found
+        assert_raises(ActiveRecord::RecordNotFound) do
+          get @attachment_url
+        end
+      end
+
+      it 'deletes attachment in Asset Manager' do
+        Services.asset_manager.expects(:delete_asset)
+          .with(asset_id)
+        AssetManagerDeleteAssetWorker.drain
       end
     end
   end