accounting: restrict restarting accounting period to Assignor
This workflow involved an Assignee who can open, close, re-open and an Assignor who can close definitively. This is usually configured so that accountants are Assignee and CFO is Assignor.
We realized that re-opening a Period that was previously closed is something we don't want the accountants to do without CFO's approval.
To support this configuration, we only allow Assignor to re-open.
Now Assignee can open and close temporarily and Assignor can re-open and close definitively.