Fix visited state

- [fix] Items with a dark background & light text have a proper visited state (instead of grey)
- [fix] Huge story (baguette) is now brand blue to follow the same colour code as the top story (fromage)
- [fix] Spacing between huge story and top story on mobile was too big

For some ridiculous reason `color: rgba(…)` won't work on visited links (at least in Webkit/Blink), so I used Sass' `mix($color-1, $color-2, $weight)` function to simulate a toned down white colour on dark backgrounds.

![ ](http://media.giphy.com/media/cC7kyUjnyj46k/giphy.gif)

![screen shot 2014-02-19 at 15 44 56](https://f.cloud.github.com/assets/85783/2208269/d35e25ca-997d-11e3-9ea6-41135a442fea.PNG)

![screen shot 2014-02-19 at 15 45 19](https://f.cloud.github.com/assets/85783/2208267/cfe5f8e6-997d-11e3-9f6b-82352dc5e034.PNG)