Generate sitemap files

Closes #623

- Add basic init and eventing scaffold
- Add sitemap-index.xml generation
- Broke out generators to individual files, added request handler
- Add page, author and tag xml files; add index mapping
- Add SiteMapManager unit tests
- Add Generators tests
- Cache invalidation headers for sitemap-*.xml
- Redirect sitemap.xml to index and rename to sitemap-index
- Handle page convert and publish/draft changes
- Add very basic functional test for route existence
- Add cache headers to sitemap routes
Merge pull request #4348 from jgable/sitemaps

Generate sitemap.xml