DEV: Introduce enable_new_notifications_menu site setting (#19860)

The `enable_new_notifications_menu` site setting allows sites that have
`navigation_menu` set to `legacy` to use the redesigned notifications
menu before switching to the new sidebar navigation menu.

lib/validators/enable_new_notifications_menu_validator.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class EnableNewNotificationsMenuValidator
+  def initialize(opts = {})
+  end
+
+  def valid_value?(value)
+    return true if value == "f"
+    SiteSetting.navigation_menu == "legacy"
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.enable_new_notifications_menu_not_legacy_navigation_menu")
+  end
+end