Redirect to workflow show page if document already signed

Verify the state of the signature. If isSigned is true, redirection to
workflow show page.

src/Bundle/ChillMainBundle/Controller/WorkflowController.php

@@ -382,6 +382,10 @@ class WorkflowController extends AbstractController
             throw new NotFoundHttpException('signature not found');
         }
 
+        if ($signature->isSigned()) {
+            return $this->redirectToRoute('chill_main_workflow_show', ['id' => $signature->getStep()->getEntityWorkflow()->getId()]);
+        }
+
         if ($signature->getSigner() instanceof User) {
             return $this->redirectToRoute('chill_main_workflow_signature_add', ['id' => $signature_id]);
         }