Add a ConceptReference endpoint to the API

In RESTWS-890, we added the capability to request multiple observations in one request via the references search parameter. This works quite well for retrieving concepts by references (so either UUID or mappings); however, often client applications need not just the list of concepts that match any of those references but they need to know which concept goes with which reference. To this end, we should create an endpoint that will search concepts by UUID or mapping and return the concept in a JSON document like:

{
    "CIEL:1234": { /* CIEL:1234 concept object */ },
    "fffa8fc5-194c-4d9a-bcc7-c8c91f922249": { /* Concept with UUID fffa8fc5-194c-4d9a-bcc7-c8c91f922249 */ }
}

This will allow the frontend to identify which concept goes with which reference.