accounting: only allow Assignor to restart accounting periods

This partially reverts 8a336dc5 (erp5_accounting: Allow Assignor manage Accounting Periods, 2024-09-16) for the restart transition, it is intentional that only Assignor can restart an accounting period that have been closed. The idea was to support a scenario where re-opening a period that was closed can not be done directly by the Assignee but needs validation from the assignor.