Add "latest changes" styling.

https://www.pivotaltracker.com/story/show/79185352

Before:
![screenshot 2014-11-07 15 01 13](https://cloud.githubusercontent.com/assets/109225/4954702/f7514e4c-668e-11e4-86b5-7d745c6f0cd2.png)

After:
![screenshot 2014-11-07 09 29 37](https://cloud.githubusercontent.com/assets/109225/4952573/3fe57a66-6676-11e4-83a3-c8e76df90825.png)

(ignore the missing beta image)
We try and use mixins to set font-sizes so that they are all responsive together. In this case this would be [`@core-14`](https://github.com/alphagov/govuk_frontend_toolkit/blob/master/stylesheets/_typography.scss#L92-L97)