DEV: Introduce legacy pageviews flag (#28748)

This hidden setting will allow us to roll out changes
to pageview UIs and reporting in a controlled way.

config/site_settings.yml

@@ -426,6 +426,9 @@ basic:
     default: ""
     max: 300
     hidden: true
+  use_legacy_pageviews:
+    default: false
+    hidden: true
 login:
   invite_only:
     refresh: true