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
It doesn't look like it's gonna be easy to use `xml(...)` to add that `<?xml-stylesheet ?>` tag.  The only thing I thought of that might work is trying to add it like they add the original declaration [from their source](https://github.com/dylang/node-xml/blob/master/lib/xml.js#L69-L70) but even that seems really hacky.

I'd sign off on your method just to get it done.