Move (don't copy) to "current" when there's only one version

For books where we only have (and only plan to have) one version, it makes sense to just have the pages available under "current" rather than also under the specific branch/version in the URL. The later pages aren't available in any version dropdowns (there aren't any version dropdowns when there's only one), so you can only get to the pages by knowing the branch name. Not only does this create nearly-duplicate pages that are never actually displayed, it probably hurts SEO.