Catalogus query interface

[lukavdplas]

The frontend should have a menu to make catalogus queries that can be sent to the backend (#139 )

[jgonggrijp]

This could be inspired by/based on the semantic query form in READ-IT.

[jgonggrijp]

Workflow as discussed on March 7 (with some details filled in by yours truly):

• Client sends a POST request to the catalog query endpoint.
• Server endpoint defers actual retrieval of records to edpop-explorer.
• All retrieved records are immediately added to the edpop:records graph if they do not already exist in there. If they do exist and the contents changed, an annotation with the changes is added to edpop:record-updates instead. See Graph and namespace organization #156.
• Deduplicated records along with any update annotations are returned to the client.