CONFIG: add 'demo' as default username/password

public/env.sample.js

@@ -2,7 +2,7 @@ const config = {
 	baseurl: 'http://localhost:4010',
 	documentserver_key: 'NqZSYTy5AFQ7MppbLW5moqpTk7u7YrNUHKYhKYuThnnya2WpCOIU694hIZT1FzYe',
 	// optional
-	default_username: '',
-	default_password: '',
+	default_username: 'demo',
+	default_password: 'demo',
 	prefersHashRouting: true
 };