Add management command for exporting of timelinelog

[alextreme]

For BD we have a management command which exports the timeline log (vips/utils/management/commands/export_timelinelog.py)

This would be useful to have within django-timeline-logger as exporting & cleaning the auditlog is otherwise something we need to do manually via a script

[sergei-maertens]

Should be easily achieved with an optional django-import-export dependency group