Fix insert into insert or update

[Pilen]

line 298 in bookkeeper.py lies.

        for debtor, share_string, share in debts:
            # NOTE: insert automaticly replaces old entries
            data.execute("INSERT INTO Debts(e_id, debtor, share_string, share) VALUES (?, ?, ?, ?)", e_id, debtor, share_string, share)

Insert no longer replaces the value after we updated to postgres.

I couldn't find any simple build in way to do this (At least not simpler than doing it in the application layer).