Puts the Official banner on everything, not just the homepage

This adds the sticky "Official" banner to every page, not just the homepage.

On the `/news` page:

![with-banner](https://cloud.githubusercontent.com/assets/4592/5210450/bd6709ea-759a-11e4-8d9c-751e60276f19.png)

On a blog post:

![banner-2](https://cloud.githubusercontent.com/assets/4592/5210456/d1f402b4-759a-11e4-8a4a-2f2873e99449.png)

Fixes #351.