FEAT: ads.txt support

Adds a plugin setting that holds text content served on a new /ads.txt route.

config/locales/server.en.yml

@@ -51,6 +51,7 @@ en:
     adsense_post_bottom_ad_sizes: "Choose your ad size"
     adsense_mobile_post_bottom_ad_size: "Choose your ad size"
     adsense_nth_post_code: "Show an ad after every N posts, where N is this value."
+    adsense_ads_txt: "Contents of your ads.txt file. More details available at <a href='https://support.google.com/adsense/answer/7532444?hl=en' target='_blank'>this Google AdSense help page</a>."
 
     amazon_through_trust_level: "Show your ads to users based on trust levels. Users with trust level higher than this value will not see ads."
     amazon_topic_list_top_src_code: "Enter src code to display at topic list top location"

plugin.rb

@@ -5,3 +5,20 @@
 # url: https://github.com/discourse/discourse-adplugin
 
 register_asset "stylesheets/adplugin.scss"
+
+after_initialize do
+  require_dependency 'application_controller'
+  class ::AdstxtController < ::ApplicationController
+    skip_before_action :check_xhr
+
+    def index
+      raise Discourse::NotFound unless SiteSetting.adsense_ads_txt.present?
+
+      render plain: SiteSetting.adsense_ads_txt
+    end
+  end
+
+  Discourse::Application.routes.append do
+    get '/ads.txt' => "adstxt#index"
+  end
+end