Add REST API URLs to portals JSON-LD

[maboukerfa]

Adds an api_url to each portal. The generated portals.json JSON-LD now exposes it as a nested https://schema.org/WebAPI node with schema:url:

"api_url": {
  "@id": "https://data.agroportal.eu",
  "@type": "https://schema.org/WebAPI"
}

Covers the 11 portals that have a public UI; the 4 "coming" entries (LovPortal, HSPortal, CHPortal, SocioPortal) have no link and are skipped.

[maboukerfa]

for the property of api_url we can use https://schema.org/url instead of http://data.bioontology.org/metadata/rest_url if we want to stick to schema.org