Add lightbox html to unauthenticated header

This fixes #4432.  The problem was that in the authenticated backbone header view, the lightbox div is there in the html, but it wasn't in the unauthenticated haml header view.  I can also move the unauthenticated view to backbone instead, we just have to move the "Sign up" and "Sign in" translations.  Also not sure about where a test for "there should be a #lightbox div" should go.