Fix loading of class descriptions
This MR is a followup MR of !984 (merged), which fixes the loading of class descriptions for every endpoint that includes a ClassRepresentation, except for the findClassById endpoint, which was already working. It also fixes some mapping bugs found in the EmbeddedStatementRepresentationAdapter and TemplateInstanceRepresentationAdapter.
I recommend reviewing individual commits. Please check whether I got all the affected places.
Edited by Marcel Konrad