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/preview.rb

@@ -81,7 +81,7 @@ module Onebox
     end
 
     def sanitize(html)
-      config = @options[:sanitize_config] || Sanitize::Config::ONEBOX
+      config = @options[:sanitize_config] || SanitizeConfig::ONEBOX
       config = config.merge(allowed_iframe_regexes: @options[:allowed_iframe_regexes])
 
       Sanitize.fragment(html, config)