FEATURE: Send a 'noindex' header in non-canonical responses (#15026)

* FEATURE: Optionally send a 'noindex' header in non-canonical responses

This will be used in a SEO experiment.

Co-authored-by: David Taylor <david@taylorhq.com>

config/site_settings.yml

@@ -1602,6 +1602,9 @@ security:
     regex: "^(Lax|Strict|Disabled|None)$"
   enable_escaped_fragments: true
   allow_index_in_robots_txt: true
+  allow_indexing_non_canonical_urls:
+    default: true
+    hidden: true
   moderators_manage_categories_and_groups: false
   moderators_change_post_ownership:
     client: true