16983 api extend wp patch for description

[``* `#16983` Extend API V3's work package PATCH endpoint for description``](https://community.openproject.org/work_packages/16983)
@hschink just FYI, but this will break on Rails 4 (#2104), causing the following error to be raised:

    arguments passed to url_for can't be handled. Please require routes or provide your own implementation

I can mix in `Rails.application.routes.url_helpers` instead, but this doesn't provide the `link_to` helper.

```diff
diff --git a/lib/api/utilities/url_helper.rb b/lib/api/utilities/url_helper.rb
index 3426522..fa7626e 100644
--- a/lib/api/utilities/url_helper.rb
+++ b/lib/api/utilities/url_helper.rb
@@ -30,8 +30,9 @@
 module API
   module Utilities
     module UrlHelper
+      extend ActiveSupport::Concern
       include OpenProject::StaticRouting::UrlHelpers
-      include ActionView::Helpers::UrlHelper
+      include Rails.application.routes.url_helpers
 
       def controller; end # The URL helpers need a controller, even if it's nil
     end
```

FWIW, I struggled quite a bit with module inheritance and using the right Helper module while working on #1550.