Fix Yoast's author sitemap generation with guest-authors

[shadyvb]

Yoast authors sitemap generation queries authors, optionally with published posts, so with Authorship activated, the sitemaps do not generate the desired outcome.

We need to detect for activation of both features, and replace the author sitemaps provider with an extended version of the class that accounts for guest authors as well.

[shadyvb]

ref https://github.com/humanmade/rappler/pull/435