Navbar text is now centered!

- Reformatted some custom Sass to use nesting properly
- Adjusted padding on navbar elements to get them to line up (ignored
brand as that will be replaced in R3 with the logo)
- Added mixins.scss to hold custom mixins. Contains a couple that can be
used for text alignment, but ended up not using them
- Made the gravatar image smaller to make it fit the navbar better