Fix missing protocol in dns-prefetch, improve code style (#8963)

Regression from #8942

app/helpers/application_helper.rb

@@ -83,7 +83,7 @@ module ApplicationHelper
   end
 
   def cdn_host
-    ENV['CDN_HOST'].presence
+    Rails.configuration.action_controller.asset_host
   end
 
   def cdn_host?
@@ -91,10 +91,10 @@ module ApplicationHelper
   end
 
   def storage_host
-    ENV['S3_ALIAS_HOST'].presence || ENV['S3_CLOUDFRONT_HOST'].presence
+    "https://#{ENV['S3_ALIAS_HOST'].presence || ENV['S3_CLOUDFRONT_HOST']}"
   end
 
   def storage_host?
-    storage_host.present?
+    ENV['S3_ALIAS_HOST'].present? || ENV['S3_CLOUDFRONT_HOST'].present?
   end
 end

app/views/layouts/embedded.html.haml

@@ -4,6 +4,12 @@
     %meta{ charset: 'utf-8' }/
     %meta{ name: 'robots', content: 'noindex' }/
 
+    - if cdn_host?
+      %link{ rel: 'dns-prefetch', href: cdn_host }/
+
+    - if storage_host?
+      %link{ rel: 'dns-prefetch', href: storage_host }/
+
     = stylesheet_pack_tag 'common', media: 'all'
     = stylesheet_pack_tag Setting.default_settings['theme'], media: 'all'
     = javascript_pack_tag 'common', integrity: true, crossorigin: 'anonymous'