import database cleanup of temp tables

[tpurschke]

• [ ] reliably delete residue in import_rule/object/service/user tables/config
• [ ] add (nightly) maintenance window in importer which may be used for cleanup jobs like this one
• [ ] add delete mutation for current Import to rollback in UI

[tpurschke]

currently importer_xxx tables do not fill up any more

[tpurschke]

these tables will soon be retired