Use semantic elements and class names for the navigation

I changed up some markup and class names in an attempt to keep the logged in and logged out versions of the navigation a little more consistent.

* Using the `<header>` and `<nav>` elements
* Unique IDs for the header and nav, because they are major layout elements that will only occur once per page
* Simplify class names: don't really need that many, as the navigation isn't complicated.

Ideally, I think the logged out and logged in headers could share a single partial/view...the only difference between them is the position of the log in/log out button, and that can be easily fixed with a class hook. 

But I figured this is a small step toward a refactor. I welcome your feedback!