Improve ActivityPub/OStatus compatibility (#4632)

*Note: OStatus URIs are invalid for ActivityPub. But we have them for
as long as we want to keep old OStatus-sourced content and as long as
we remain OStatus-compatible.*

- In Announce handling, if object URI is not a URL, fallback to object URL
- Do not use specialized ThreadResolveWorker, rely on generalized handling
- When serializing notes, if parent's URI is not a URL, use parent's URL

app/lib/activitypub/activity/create.rb

@@ -91,7 +91,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
   def resolve_thread(status)
     return unless status.reply? && status.thread.nil?
-    ActivityPub::ThreadResolveWorker.perform_async(status.id, @object['inReplyTo'])
+    ThreadResolveWorker.perform_async(status.id, @object['inReplyTo'])
   end
 
   def conversation_from_uri(uri)