Redirect countdown <= 1.0 - Cross-Site Request Forgery to Settings Update

The Redirect countdown plugin for WordPress is vulnerable to Cross-Site Request Forgery in all versions up to, and including, 1.0. This is due to missing nonce validation on the `countdown_settings_content()` function. This makes it possible for unauthenticated attackers to update the plugin settings including the countdown timeout, redirect URL, and custom text, via a forged request granted they can trick a site administrator into performing an action such as clicking on a link.