Mbst 9478 sitemap improvements

1. Allow the base URI to be provided on the request, rather than just the hostname. This allows us to switch the proxying method while still supporting the old api
1. Use URI instead of curie, if there's no curie set
1. Output content and video URIs according to publisher-specific scheme
Should this be at model level? i.e. should a content be able to provide its own top-level uri?