Fix too many forwards (#5854)

* Avoid sending explicit Undo->Announce when original deleted

* Do not forward a reply back to the server that sent it

* Deduplicate inboxes of rebloggers' followers for delete forwarding

* Adjust test

* Fix wrong class, bad SQL, wrong variable, outdated comment

app/models/account.rb

@@ -214,6 +214,10 @@ class Account < ApplicationRecord
     Rails.cache.fetch("exclude_domains_for:#{id}") { domain_blocks.pluck(:domain) }
   end
 
+  def preferred_inbox_url
+    shared_inbox_url.presence || inbox_url
+  end
+
   class << self
     def readonly_attributes
       super - %w(statuses_count following_count followers_count)