Disable secret key validation on admin noroute to fix redirect loop

Similar to 2f422ad but using the framework.

Fixes issue #10611 for the `2.2-develop` branch.

Doesn't address the route cause, but only fixes the symptom as per #10921 (comment)

Author: convenient

app/code/Magento/Backend/Controller/Adminhtml/Noroute/Index.php

@@ -8,6 +8,13 @@
 
 class Index extends \Magento\Backend\App\Action
 {
+    /**
+     * Array of actions which can be processed without secret key validation
+     *
+     * @var string[]
+     */
+    protected $_publicActions = ['index'];
+
     /**
      * @var \Magento\Framework\View\Result\PageFactory
      */