chore(styles): configure sass directories

Also,  I think I mentioned previously, but you can remove the leading underscores in your imports here.

This worked for me locally:

```css
@import 'modules/container';
@import 'modules/global-nav';
```

Not that you don't believe me, but you can find some details in the "Partials" and "Import" sections at http://sass-lang.com/guide#topic-4