Added the unprotected root action support

Needed this for [Gitlab Hook Plugin](https://github.com/elvanja/jenkins-gitlab-hook-plugin) where the hook needs to be accessible anonymously. Currently fixed directly in the plugin itself: https://github.com/elvanja/jenkins-gitlab-hook-plugin/blob/master/models/gitlab_web_hook_root_action.rb#L3
But, it would be nice to have the support directly.