Change grouped notifications API shape (take 2) (#31214)

app/serializers/rest/dedup_notification_group_serializer.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class REST::DedupNotificationGroupSerializer < ActiveModel::Serializer
+  has_many :accounts, serializer: REST::AccountSerializer
+  has_many :statuses, serializer: REST::StatusSerializer
+  has_many :notification_groups, serializer: REST::NotificationGroupSerializer
+end