Use Plek to get website_root in sitemap generator
Generate sitemap files under $docroot/sitemaps/

This is so that these paths can be routed without requiring regex based
routes.

The base sitemap.xml symlink is still created at $docroot/sitemap.xml.
Merge pull request #170 from alphagov/update_sitemap_paths

Generate sitemap files under $docroot/sitemaps/