Facia tool: 'stale front' alerts

Shows the selected front's age since last pressing:

![fronts-tool](https://cloud.githubusercontent.com/assets/1147913/4188312/3f4f20f4-3771-11e4-9eea-b7875a5f74b5.png)

Alerts if that front is older than twice it's cron rate (assumes differential rates from PR https://github.com/guardian/frontend/pull/5876):

![fronts-tool-2](https://cloud.githubusercontent.com/assets/1147913/4189370/a92ef2e2-377b-11e4-8ab2-97c043b25da5.png)