When a field or a model is unlinked, keep the related audit logs. Denormalize
the field and model info on the logs and log lines so that the information
is still available after the deletion of the related data model.
Also, to improve the performance of the deletion of fields and models,
add indexes on the log's model_id and log line's field_id.
Co-Authored-By: Stefan Rijnhart <stefan@opener.am>