OSRA273 fix duplicate osra_num on import

OSRA273 fix duplicate osra_num on import
I understand the rationale behind setting `seq_id = nil`, but not behind replacing `before_create` with `before_save`. The two callbacks should fire at ~exactly the same time on the first save to the db, and using `before_create` does not force `osra_num` recalculation on every subsequent save.