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
Considered that, but decided that as it was already instantiated in-class, and I wasn't going to retrofit tests to the class at this stage, I didn't change.