Adds some icons to the navbar!

- Adds icon SVG files
- Reduces size of links that are not currently icons
- Centers the navbar column
- Resizes the user’s avatar to the size of the column
- Removes borders from links in the navbar
- Adds 10px margins between links in the navbar

TODO:
- Add icons for Help and Logout
- Add icon font for color-based hover states
- Strangle Adobe Illustrator
- Make margin and padding OOCSS, yo.