Add likes to activitypub object representation

Top level of the likes OrderedCollection is inlined to get immediate
access to totalItems. Because the count can be returned without scanning
the database for like activities the extra query is saved when the
client only wants to display the total.

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -629,6 +629,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     |> add_mention_tags
     |> add_emoji_tags
     |> add_attributed_to
+    |> add_likes
     |> prepare_attachments
     |> set_conversation
     |> set_reply_to_uri
@@ -788,6 +789,22 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     |> Map.put("attributedTo", attributedTo)
   end
 
+  def add_likes(%{"id" => id, "like_count" => likes} = object) do
+    likes = %{
+      "id" => "#{id}/likes",
+      "first" => "#{id}/likes?page=1",
+      "type" => "OrderedCollection",
+      "totalItems" => likes
+    }
+
+    object
+    |> Map.put("likes", likes)
+  end
+
+  def add_likes(object) do
+    object
+  end
+
   def prepare_attachments(object) do
     attachments =
       (object["attachment"] || [])
@@ -803,7 +820,6 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
   defp strip_internal_fields(object) do
     object
     |> Map.drop([
-      "likes",
       "like_count",
       "announcements",
       "announcement_count",