Add human readable toString implementations into the data model

The audit trails are simply calling toString on menas entities to generate the old and new values.

Provide human-readable implementation of these values to improve the user experience.

Start by locating the data model entities whose toString output is used by the audit trails, then inspect how their current values appear in old and new audit entries. The work is done when those entries show human-readable representations instead of the existing default values.

Vom Indexierungsmodell aus dem Issue-Text verfasst.