FEATURE: Add dash warning for using S3 without CDN (#11561)

Over the years we had some admins complaining that most of their costs
running Discourse is on S3 uploads.

That only happens when they are using S3 without a CDN, which is widely
considered a bad pratice for public websites, as egress bandwidth can
get expensive quite fast.

Related:

https://meta.discourse.org/t/download-avatars-locally-from-s3-shutting-down-s3-in-my-site/174001/3?u=falco

https://meta.discourse.org/t/use-webtorrent-to-load-media-objects/155873/2?u=falco

app/models/admin_dashboard_data.rb

@@ -92,7 +92,7 @@ class AdminDashboardData
     add_problem_check :rails_env_check, :host_names_check, :force_https_check,
                       :ram_check, :google_oauth2_config_check,
                       :facebook_config_check, :twitter_config_check,
-                      :github_config_check, :s3_config_check,
+                      :github_config_check, :s3_config_check, :s3_cdn_check,
                       :image_magick_check, :failing_emails_check,
                       :subfolder_ends_in_slash_check,
                       :pop3_polling_configuration, :email_polling_errored_recently,
@@ -190,6 +190,12 @@ class AdminDashboardData
     nil
   end
 
+  def s3_cdn_check
+    if (GlobalSetting.use_s3? || SiteSetting.enable_s3_uploads) && SiteSetting.Upload.s3_cdn_url.blank?
+      I18n.t('dashboard.s3_config_warning')
+    end
+  end
+
   def image_magick_check
     I18n.t('dashboard.image_magick_warning') if SiteSetting.create_thumbnails && !system("command -v convert >/dev/null;")
   end

config/locales/server.en.yml

@@ -1417,6 +1417,7 @@ en:
     github_config_warning: 'The server is configured to allow signup and login with GitHub (enable_github_logins), but the client id and secret values are not set. Go to <a href="%{base_path}/admin/site_settings">the Site Settings</a> and update the settings. <a href="https://meta.discourse.org/t/configuring-github-login-for-discourse/13745" target="_blank">See this guide to learn more</a>.'
     s3_config_warning: 'The server is configured to upload files to S3, but at least one the following setting is not set: s3_access_key_id, s3_secret_access_key, s3_use_iam_profile, or s3_upload_bucket. Go to <a href="%{base_path}/admin/site_settings">the Site Settings</a> and update the settings. <a href="https://meta.discourse.org/t/how-to-set-up-image-uploads-to-s3/7229" target="_blank">See "How to set up image uploads to S3?" to learn more</a>.'
     s3_backup_config_warning: 'The server is configured to upload backups to S3, but at least one the following setting is not set: s3_access_key_id, s3_secret_access_key, s3_use_iam_profile, or s3_backup_bucket. Go to <a href="%{base_path}/admin/site_settings">the Site Settings</a> and update the settings. <a href="https://meta.discourse.org/t/how-to-set-up-image-uploads-to-s3/7229" target="_blank">See "How to set up image uploads to S3?" to learn more</a>.'
+    s3_cdn_warning: 'The server is configured to upload files to S3, but there is no S3 CDN configured. This can lead to expensive S3 costs and slower site performance. <a href="https://meta.discourse.org/t/-/148916" target="_blank">See "Using Object Storage for Uploads" to learn more</a>.'
     image_magick_warning: 'The server is configured to create thumbnails of large images, but ImageMagick is not installed. Install ImageMagick using your favorite package manager or <a href="https://www.imagemagick.org/script/download.php" target="_blank">download the latest release</a>.'
     failing_emails_warning: 'There are %{num_failed_jobs} email jobs that failed. Check your app.yml and ensure that the mail server settings are correct. <a href="%{base_path}/sidekiq/retries" target="_blank">See the failed jobs in Sidekiq</a>.'
     subfolder_ends_in_slash: "Your subfolder setup is incorrect; the DISCOURSE_RELATIVE_URL_ROOT ends in a slash."