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/add_serializer.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class ActivityPub::AddSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :type, :actor, :target
+  attribute :proper_object, key: :object
+
+  def type
+    'Add'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+
+  def proper_object
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def target
+    account_collection_url(object, :featured)
+  end
+end