Support more variations of ActivityPub keyId in signature (#4630)

- Tries to avoid performing HTTP request if the keyId is an actor URI
- Likewise if the URI is a fragment URI on top of actor URI
- Resolves public key, returns owner if the owner links back to the key

app/lib/activitypub/activity.rb

@@ -58,7 +58,7 @@ class ActivityPub::Activity
   end
 
   def object_uri
-    @object_uri ||= @object.is_a?(String) ? @object : @object['id']
+    @object_uri ||= value_or_id(@object)
   end
 
   def redis