Identify primary topic of JSON-LD response (see #257 and #230)

Use JSON-LD `@id` keyword and `foaf:primaryTopic` property to
identify the main resource that a (multi-entity, `@graph`) JSON-LD
response is about (e.g. the resource, the person, etc.)
Fix type information in views: use type of primary topic (see #257)
Only add primary topic info if there are multiple entities (#257)
Merge pull request #306 from fsteeg/issue-257-topic

Identify primary topic in JSON-LD response and fix type information in views