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

app/presenters/grouped_notifications_presenter.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class GroupedNotificationsPresenter < ActiveModelSerializers::Model
+  def initialize(grouped_notifications)
+    super()
+
+    @grouped_notifications = grouped_notifications
+  end
+
+  def notification_groups
+    @grouped_notifications
+  end
+
+  def statuses
+    @grouped_notifications.filter_map(&:target_status).uniq(&:id)
+  end
+
+  def accounts
+    @grouped_notifications.flat_map(&:sample_accounts).uniq(&:id)
+  end
+end