Add XSL for Sitemaps

fixes #4555

- There's no easy way to declare an XSL with the node xml module, so I
  needed to move the declarations to both be strings
- Ideally the code to serve the XSL would also be inside the sitemap
  module, but I think we need to refactor a bit to get there easily
- Added the XSL from #4559, with minor amends to make the tables and urls
  display correctly
Haha none, I was daydreaming about how to solve the XSL and it just happened :P