Add experimental server-side notification grouping (#29889)

app/models/notification_group.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class NotificationGroup < ActiveModelSerializers::Model
+  attributes :group_key, :sample_accounts, :notifications_count, :notification
+
+  def self.from_notification(notification)
+    if notification.group_key.present?
+      # TODO: caching and preloading
+      sample_accounts = notification.account.notifications.where(group_key: notification.group_key).order(id: :desc).limit(3).map(&:from_account)
+      notifications_count = notification.account.notifications.where(group_key: notification.group_key).count
+    else
+      sample_accounts = [notification.from_account]
+      notifications_count = 1
+    end
+
+    NotificationGroup.new(
+      notification: notification,
+      group_key: notification.group_key || "ungrouped-#{notification.id}",
+      sample_accounts: sample_accounts,
+      notifications_count: notifications_count
+    )
+  end
+
+  delegate :type,
+           :target_status,
+           :report,
+           :account_relationship_severance_event,
+           to: :notification, prefix: false
+end