4.0.0-alpha.6: Navbar should have style of .navbar-toggleable-xl by default

Created by: spongessuck

My navbar is compact and shouldn't be toggleable. I've had to set the class to .navbar-toggleable-xl to make this work, which is counter-intuitive.

I don't understand why a navbar-toggleable-* is required when .navbar-toggleable-xl makes it never toggle. Navbars should have the style currently given to navbar-toggleable-xl by default.