Add noop relay backend

Part of cloudron/box#622

src/views/email.js

@@ -389,6 +389,7 @@ angular.module('Application').controller('EmailController', ['$scope', '$locatio
         { provider: 'mailjet-smtp', name: 'Mailjet', host: '', port: 587 },
         { provider: 'postmark-smtp', name: 'Postmark', host: 'smtp.postmarkapp.com', port: 587 },
         { provider: 'sendgrid-smtp', name: 'SendGrid', host: 'smtp.sendgrid.net', port: 587, username: 'apikey' },
+        { provider: 'noop', name: 'Disable' },
     ];
 
     $scope.mailRelay = {