Distinguish between Whitehall environments

* Distinguish between Preview and Production so that users don't accidentally make changes to Production content whilst under the assumption they're working on Preview.
* Highlight environment using a favicon and a coloured label
* Will need changes to deploy scripts (see https://github.gds/gds/alphagov-deployment/pull/603)

![screen shot 2014-06-11 at 16 02 24](https://cloud.githubusercontent.com/assets/319055/3246221/74521bf0-f179-11e3-9942-10542dbc6fd4.png)