DEV: Hide admin Moderation Flags UI behind feature flag for now (#27756)

Adds experimental_flags_admin_page_enabled_groups (default "")
to remove the Moderation Flags link from the admin sidebar for now,
there are still a few bugfixes that need to be done before we
are comfortable with turning this on more widely. This is
a _temporary_ flag, we will be removing this once the feature
is more stable.

config/site_settings.yml

@@ -2427,6 +2427,12 @@ developer:
     list_type: compact
     allow_any: false
     refresh: true
+  experimental_flags_admin_page_enabled_groups:
+    default: ""
+    type: group_list
+    list_type: compact
+    allow_any: false
+    refresh: true
 
 navigation:
   navigation_menu: