Support semantic web standard for linked websites

[mpgirro]

Feeds are often quite scarce with data. Yet podcasts and episodes they link to websites, which hold more data. We already support downloading and adding these websites to the Solr index.

Websites can also use semantic web standard. We should extract and handle them separately, to improve search quality.

See [schema.org] (https://schema.org/docs/gs.html) for introduction and general information.

Basically, there are these 3 format standards:

• Microdata
• RDFa
• JSON-LD