Distinguish between 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, a coloured navigation bar and a coloured label
* Determine environment based on variable set at deploy time (see https://github.gds/gds/alphagov-deployment/pull/603 for an example)
* Based on https://github.com/alphagov/publisher/pull/203
* Namespace new images
* Allow apps to override default favicons

![screen shot 2014-06-04 at 16 30 03](https://cloud.githubusercontent.com/assets/319055/3175995/b78bd844-ec00-11e3-806e-531f6367b517.png)