Federate pinned statuses over ActivityPub (#6610)

* Federate pinned statuses over ActivityPub

* Display pinned toots in web UI

Fix #6117

* Fix migration

* Fix tests

* Update outbox_serializer.rb

* Update remove_serializer.rb

* Update add_serializer.rb

* Update fetch_featured_collection_service.rb

app/serializers/activitypub/actor_serializer.rb

@@ -4,7 +4,7 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :id, :type, :following, :followers,
-             :inbox, :outbox,
+             :inbox, :outbox, :featured,
              :preferred_username, :name, :summary,
              :url, :manually_approves_followers
 
@@ -53,6 +53,10 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
     account_outbox_url(object)
   end
 
+  def featured
+    account_collection_url(object, :featured)
+  end
+
   def endpoints
     object
   end