Add IRI for namespace in RDF serialisations

Hi. The RDF files do not specify what is the IRI of the vocab in the file itself. e.g. interop.ttl does not specify namespace IRI in the prefix (<#>) or the vocabulary description (<>) or the definition of terms (rdfs:isDefinedBy <>). So once downloaded, there is no way to figure out what's the IRI. Though there is a prefix interop, it cannot be assumed to be the IRI.

PREFIX : <#>
PREFIX interop: <http://www.w3.org/ns/solid/interop#>

<>
dcterms:issued "2020-06-14"^^xsd:date ;
dcterms:modified "2020-09-24"^^xsd:date ;
rdfs:label "Terms for application interoperability in Solid"@en .

:Agent
  a rdfs:Class ;
  rdfs:comment "An agent profile for the Solid Ecosystem"@en ;
  rdfs:isDefinedBy <> ;
  rdfs:label "Agent"@en .

Start by inspecting interop.ttl and the other RDF serialisations described in the issue. Check how the namespace IRI is represented in each file, then update the serialisations so a downloaded file identifies its vocabulary and terms unambiguously. Done means the namespace IRI is present consistently in the relevant prefix, vocabulary description, or term definitions.