json-ld: only add items to context if they're used in this document

[alastair]

We have some hard-coded url -> prefixes for the contexts that we use:

ce-api/src/routes/helpers/transformers.js

Lines 6 to 12 in 1e669bf

	const scopedContexts = {
	dc: 'http://purl.org/dc/terms/',
	prov: 'http://www.w3.org/ns/prov#',
	skos: 'http://www.w3.org/2004/02/skos/core#',
	rdf: 'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
	bib: 'https://bib.schema.org/'
	}

We should only return these mappings if the prefix is used in the document that is being returned