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

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

	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/'
	}