Remove the tabindex attribute on blur

This is following some feedback from the GDS Accessibility clinic: #1999 (comment)
There is no need for the component to be focusable after the page has loaded.

Only remove the tabindex attribute if it was set by JavaScript.

src/govuk/components/notification-banner/notification-banner.js

@@ -37,8 +37,14 @@ NotificationBanner.prototype.setFocus = function () {
   }
 
   // Set tabindex to -1 to make the element focusable with JavaScript.
+  // Remove the tabindex on blur as the component doesn't need to be focusable after the page has
+  // loaded.
   if (!$module.getAttribute('tabindex')) {
     $module.setAttribute('tabindex', '-1')
+
+    $module.addEventListener('blur', function () {
+      $module.removeAttribute('tabindex')
+    })
   }
 
   $module.focus()