Add a data model to hold the concepts of the EO4GEO BoK

[nuest]

http://www.eo4geo.eu/bok/

concepts: https://eo4geo-bok.firebaseio.com/.json

When contributing metadata about a work, then there should be an interactive UI to add concepts from the BoK.

The articles belonging to one concept should be listed on pages and accessible through API endpoints.