Fix loading of predicate descriptions

This MR fixes the loading of predicate descriptions for every endpoint that includes a PredicateRepresentation, except for the findPredicateById endpoint, which was already working. The issue was caused by the queries itself, because the description statement of a predicate was not fetched at all, because it was simply forgotten. We decided to fix this bug by deferring the loading of the descriptions in the representation adapters, which effectively meant removing the description field of the Predicate domain model class. Because of this, the description field of the PredicateReference model class had to be removed as well, as its only purpose was to improve performance by not including fields that are not directly available from the Thing model instance (for example, the shared property of resources). The removal of said field was approved by the front-end team, but also required ThingReferenceRepresentation classes for the deserialization in tests.

The same problem with the descriptions also applies to classes, which will be handled in a followup MR.

Since I expected a lot of code changes with this MR, I tried to keep the commits somewhat review-friendly. I recommend reviewing individual commits. Please check whether I got all the affected places.