AVG Timeline / Audit logs - print repr of object instead of IDs

[sergei-maertens]

Split off from #1408 / #1409

The audit/AVG logs show the IDs of objects that are the "subject" of the logged action. This is workable for objects that still exist, but not so much for records that have been deleted (which happens, otherwise #1408 would not be an issue).

Instead of printing the ID, both the representation and the ID of the object should be displayed, see #1409 (review)

Tasks

• On every log action, store the str(object) in the database recorded (can be recorded in extra_data dict)
• In the fmt_lead, always display the object representation if available, and include the object ID as well, e.g.: Formulier "toeslag aanvragen" (ID: 42)