minor #13363 Move login route to const (JakubSzczesniak)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead (closes #13363).

Discussion
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Move login route to const

This is more stable solution. I spent half of hour to fix bug, because I forgot change route name in 2 places.

Commits
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5b0a583 Update form_login_setup.rst

Author: javiereguiluz

security/form_login_setup.rst

@@ -191,6 +191,8 @@ a traditional HTML form that submits to ``/login``:
     class LoginFormAuthenticator extends AbstractFormLoginAuthenticator implements PasswordAuthenticatedInterface
     {
         use TargetPathTrait;
+        
+        public const LOGIN_ROUTE = 'app_login';
 
         private $entityManager;
         private $urlGenerator;
@@ -207,7 +209,7 @@ a traditional HTML form that submits to ``/login``:
 
         public function supports(Request $request)
         {
-            return 'app_login' === $request->attributes->get('_route')
+            return self::LOGIN_ROUTE === $request->attributes->get('_route')
                 && $request->isMethod('POST');
         }
 
@@ -260,7 +262,7 @@ a traditional HTML form that submits to ``/login``:
 
         protected function getLoginUrl()
         {
-            return $this->urlGenerator->generate('app_login');
+            return $this->urlGenerator->generate(self::LOGIN_ROUTE);
         }
     }