Add option to request partial accounts in grouped notifications API (#31299)

app/serializers/rest/dedup_notification_group_serializer.rb

@@ -1,7 +1,22 @@
 # frozen_string_literal: true
 
 class REST::DedupNotificationGroupSerializer < ActiveModel::Serializer
+  class PartialAccountSerializer < REST::AccountSerializer
+    # This is a hack to reset ActiveModel::Serializer internals and only expose the attributes
+    # we care about.
+    self._attributes_data = {}
+    self._reflections = []
+    self._links = []
+
+    attributes :id, :acct, :locked, :bot, :url, :avatar, :avatar_static
+  end
+
   has_many :accounts, serializer: REST::AccountSerializer
+  has_many :partial_accounts, serializer: PartialAccountSerializer, if: :return_partial_accounts?
   has_many :statuses, serializer: REST::StatusSerializer
   has_many :notification_groups, serializer: REST::NotificationGroupSerializer
+
+  def return_partial_accounts?
+    instance_options[:expand_accounts] == 'partial_avatars'
+  end
 end