Add isPrimaryTopicOf to webid-profile-data-model writer-application-link-extended-profile-webid

[csarven]

If my brief research and memory serve me well, foaf:isPrimaryTopicOf was touched upon in discussions but I think defaulted to continue using rdfs:seeAlso in the Solid WebID Profile data model.

It is useful for profiles to refer to both to provide a broader view of an agent's profile. They have some overlap but in the context of the subject (the WebID), social agents may want to refer to anything for that matter that is useful in different contexts. In order to enable wider use by applications, we should encourage both.

[TallTed]

Yeah. rdfs:seeAlso is VERY commonly used, but offers such a tenuous semantic connection, it's little better than an untyped a href link. foaf:isPrimaryTopicOf is a much stronger and specific type of connection.