Calculate optimal schema

This might need to be a more sophisticated algorithm in the future, but for now we basically need to implement a method that takes the list of queried provider ids and returns the schema uuid of the mostly used schema.

Nice touch: Show usage of the schema in UI.

(from redmine: issue id 42, created on 2018-01-22tgradl, closed on 2018-01-22)

• Relations:
  • relates #13 (closed)
  • relates #44 (closed)
  • relates #45 (closed)
  • relates #52 (closed)
  • relates #53 (closed)