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.