Update schema.org markup to match current Dataset spec and best practices

[philipashlock]

When schema.org markup for Dataset was first incorporated back in 2014 (via #327 & #652 ) the Google Webmaster tools showed it was working well, but it looks like the schema.org mapping to DCAT has evolved a bit since then and Google Search Console is now showing a number of errors and warnings.

We may also want to look into moving the schema.org support out of the template markup microdata and into JSON-LD and future proof the implementation in line with other more widespread CKAN implementations (e.g. ckanext-dcat's support)

We can also discuss in the comments below how we might want to fine tune the implementation, e.g. we received a recommendation to use sameAs to make it clear when we're syndicating a metadata record published elsewhere (e.g. from an agency's own web-based metadata catalog entry or from a state or local catalog).

For Example:
This dataset: https://catalog.data.gov/dataset/congress-district-breakdowns-35276 would benefit from having "sameAs" field set to https://data.cityofnewyork.us/d/77d2-9ebr.