Refactor navbar to eliminate nested {{link-to}}.

This is essentially setting up a binding from the `LinkView`'s `active` status and whatever is set as `alternativeActive` in the `{{link-to}}` (in our case the `activating-list-item` component.

Here is a sample [JSBin](http://emberjs.jsbin.com/navew/2/edit) demoing the technique.

Screenshots showing that the proper CSS classes are given:

![screenshot](http://monosnap.com/image/tzOH6n82rIVGNEFAUWVOLN52QuASQ4.png)

![screenshot](http://monosnap.com/image/5gmcwJcj0kgEXt8lnET4OKgIZ8KRpH.png)

![screenshot](http://monosnap.com/image/UCVNukTXLMNfVneLhzwVyhkrVlGSBt.png)