FEATURE: let Google index pages so it can remove them

Google insists on indexing pages so it can figure out if they
can be removed from the index.

see: https://support.google.com/webmasters/answer/6332384?hl=en

This change ensures the we have special behavior for Googlebot
where we allow indexing, but block the actual indexing via
X-Robots-Tag

spec/requests/topics_controller_spec.rb

@@ -1853,7 +1853,7 @@ RSpec.describe TopicsController do
 
       get "/t/#{topic.slug}/#{topic.id}.json"
 
-      expect(response.headers['X-Robots-Tag']).to eq('noindex')
+      expect(response.headers['X-Robots-Tag']).to eq('noindex, nofollow')
     end
 
     it "doesn't store an incoming link when there's no referer" do