FIX: Render links with subfolders properly in Discobot

Currently, some links aren’t properly built in Discobot when Discourse
is hosted in a subfolder. This is because we’re providing
`Discourse.base_url` to the Rails helpers which contains the base URL
*with* the prefix. But Rails helpers already handle this prefix so the
resulting link gets the prefix twice.

The fix is quite simple: use `Discourse.base_url_no_prefix` instead of
`Discourse.base_url`.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/new_user_narrative.rb

@@ -638,7 +638,10 @@ module DiscourseNarrativeBot
     end
 
     def url_helpers(url, opts = {})
-      Rails.application.routes.url_helpers.public_send(url, opts.merge(host: Discourse.base_url))
+      Rails.application.routes.url_helpers.public_send(
+        url,
+        opts.merge(host: Discourse.base_url_no_prefix),
+      )
     end
   end
 end