Fix hamburger button with thin screen

[sauln]

If you decrease the width of the browser, the nav-bar automatically collapses into a hamburger button. The hamburger button doesn't do anything though, so we can't access the navigation keys.

Fix the hamburger button is it creates a drop down navigation panel.

[missygerlach]

We now have a fully-functional hamburger that has real buttons and that actually do things !

[accua]

Nice!