Always explicitly identify the primary topic

See #315.

We can safely review and merge this. Before deploying however, we should closely coordinate with @literarymachine in #315, since this alters the JSON-LD in a way that causes issues in the current @edoweb deployment (needs to switch to using N-Triples directly when we deploy this).

Deployed to staging:

http://staging.api.lobid.org/person?id=171339061&format=full

`curl -H "Accept: text/plain" "http://staging.api.lobid.org/person?id=171339061"`

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