Fronts: image override

Facia:

![fronts-tool](https://cloud.githubusercontent.com/assets/1147913/3025924/313d6d34-e00c-11e3-80a2-a2f8fa5a9555.png)

Facia tool:

![fcd417cc-db7a-11e3-9a5a-8a988ca7e13e](https://cloud.githubusercontent.com/assets/1147913/3026093/b196622c-e00e-11e3-94dd-99fb68e56464.png)


Only accepts URLs for images that satisfy: 
* 5x3 ratio
* 620 <= width <= 940
* hosted on *.guim.co.uk

Until we have a media service, such images can be cropped/uploaded using the Batch Uploader tool.