Flesh out schema.org microdata

[mdrum]

Google Dataset search uses schema.org to scrape structured metadata from pages on the web and perform searches against them. I initially added some of this to the dataset page, but have not gone back in several months to finish it up.

The schema can be found here: https://schema.org/Dataset
Anything that we have available in the registry, or Archive Loader, should be marked up with this schema. Even if the fields are not being displayed visually, we can include them as hidden metadata specifically for the crawler.