Fix dereferencing remote statuses not using the correct account

Follow-up to mastodon#14359

In the case of limited toots, the receiver may not be explicitly part of the
audience. If a specific user's inbox URI was specified, it makes sense to
dereference the toot from the corresponding user, instead of trying to find
someone in the explicit audience.

app/lib/activitypub/activity.rb

@@ -168,6 +168,8 @@ def dereference_object!
   end
 
   def signed_fetch_account
+    return Account.find(@options[:delivered_to_account_id]) if @options[:delivered_to_account_id].present?
+
     first_mentioned_local_account || first_local_follower
   end