Block document editing if any signature associated to a workflow is signed

Add a check in `WorkflowStoredObjectPermissionHelper` to block document editing once any signature is signed. Accompanied by new tests to verify this behavior.

src/Bundle/ChillDocStoreBundle/Service/WorkflowStoredObjectPermissionHelper.php

@@ -11,6 +11,7 @@ declare(strict_types=1);
 
 namespace Chill\DocStoreBundle\Service;
 
+use Chill\MainBundle\Entity\Workflow\EntityWorkflowSignatureStateEnum;
 use Chill\MainBundle\Workflow\EntityWorkflowManager;
 use Symfony\Component\Security\Core\Security;
 
@@ -31,6 +32,16 @@ class WorkflowStoredObjectPermissionHelper
             if (!$workflow->getCurrentStep()->getAllDestUser()->contains($currentUser)) {
                 return false;
             }
+
+            // as soon as there is one signatured applyied, we are not able to
+            // edit the document any more
+            foreach ($workflow->getSteps() as $step) {
+                foreach ($step->getSignatures() as $signature) {
+                    if (EntityWorkflowSignatureStateEnum::SIGNED === $signature->getState()) {
+                        return false;
+                    }
+                }
+            }
         }
 
         return true;