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.

/reviewed-on nexedi/erp5!813

bt5/erp5_accounting/WorkflowTemplateItem/portal_workflow/accounting_period_workflow/transitions/restart_action.xml

@@ -65,7 +65,7 @@
             <key> <string>roles</string> </key>
             <value>
               <tuple>
-                <string>Assignee</string>
+                <string>Assignor</string>
               </tuple>
             </value>
         </item>