External links are not rendered with the correct css class

Reproduction steps

1. Connect to a backend that does not return the HTML rendered content from the server (i.e., any backend except XWiki)
2. Create a page with two links, one internal and one external
3. Save and view the page

Expected

The external page link is wrapped in a span with class  wikiexternallink

Actual

No links are marked as external. Which is easily explained by the lack of support for this feature in the markdown to HTML rendering code.