Add a way to hide the menu when not logged in

Moved from GitHub #199:

Hi! WriteFreely is super nice and clean, however I’d like the Menu to be hidden when I’m not logged in, so the people visiting my blog won’t see it. I don’t mind manualy adding /login to the URL when I need to log in. Is this possible? Thanks!

To do this on a single-user WriteFreely instance, you’ll just add a little custom CSS from your blog’s Customize page:

body#collection nav#manage {
    display: none;