DEV: Don’t patch Sanitize::Config

Currently we’re reopening the `Sanitize::Config` class (which is part of
the `sanitize` gem) to put our custom config for Onebox in it. This is
unnecessary as we can simply create a dedicated module to hold our
custom configuration.

lib/onebox.rb

@@ -20,7 +20,7 @@ module Onebox
     load_paths: [File.join(Rails.root, "lib/onebox/templates")],
     allowed_ports: [80, 443],
     allowed_schemes: ["http", "https"],
-    sanitize_config: Sanitize::Config::ONEBOX,
+    sanitize_config: SanitizeConfig::ONEBOX,
     redirect_limit: 5
   }