Serialize ActivityPub alternate link into OStatus deletes, handle it (#4730)

Requires moving Atom rendering from DistributionWorker (where
`stream_entry.status` is already nil) to inline (where
`stream_entry.status.destroyed?` is true) and distributing that.

Unfortunately, such XML renderings can no longer be easily chained
together into one payload of n items.

app/lib/ostatus/atom_serializer.rb

@@ -79,6 +79,9 @@ class OStatus::AtomSerializer
 
     if stream_entry.status.nil?
       append_element(entry, 'content', 'Deleted status')
+    elsif stream_entry.status.destroyed?
+      append_element(entry, 'content', 'Deleted status')
+      append_element(entry, 'link', nil, rel: :alternate, type: 'application/activity+json', href: ActivityPub::TagManager.instance.uri_for(stream_entry.status)) if stream_entry.account.local?
     else
       serialize_status_attributes(entry, stream_entry.status)
     end