Revert "Remove conversation URI (#11423)" (#11424)

This reverts commit 75f7f9930e.

app/serializers/activitypub/note_serializer.rb

@@ -1,13 +1,14 @@
 # frozen_string_literal: true
 
 class ActivityPub::NoteSerializer < ActivityPub::Serializer
-  context_extensions :atom_uri, :sensitive,
+  context_extensions :atom_uri, :conversation, :sensitive,
                      :hashtag, :emoji, :focal_point, :blurhash
 
   attributes :id, :type, :summary,
              :in_reply_to, :published, :url,
              :attributed_to, :to, :cc, :sensitive,
-             :atom_uri, :in_reply_to_atom_uri
+             :atom_uri, :in_reply_to_atom_uri,
+             :conversation
 
   attribute :content
   attribute :content_map, if: :language?
@@ -109,6 +110,16 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
     OStatus::TagManager.instance.uri_for(object.thread)
   end
 
+  def conversation
+    return if object.conversation.nil?
+
+    if object.conversation.uri?
+      object.conversation.uri
+    else
+      OStatus::TagManager.instance.unique_tag(object.conversation.created_at, object.conversation.id, 'Conversation')
+    end
+  end
+
   def local?
     object.account.local?
   end