FEATURE: Block indexing the embed topic list (#16495)

This adds a robots tag header to disallow indexing the topic list that
powers our embed topic list feature. It also uses a new tag that allows
the content to be indexed in the parent page.

See https://developers.google.com/search/blog/2022/01/robots-meta-tag-indexifembedded

https://meta.discourse.org/t/-/125911/127

app/controllers/embed_controller.rb

@@ -35,6 +35,8 @@ class EmbedController < ApplicationController
       raise Discourse::InvalidParameters.new(:embed_class) unless @embed_class =~ /^[a-zA-Z0-9\-_]+$/
     end
 
+    response.headers['X-Robots-Tag'] = 'noindex, indexifembedded'
+
     if params.has_key?(:template) && params[:template] == "complete"
       @template = "complete"
     else