Facia tool: 'stale front' alerts

Shows the selected front's age since last pressing. The retry button only shows if the front is older than twice it's cron refresh rate (assumes differential rates from PR https://github.com/guardian/frontend/pull/5876):

![fronts-tool](https://cloud.githubusercontent.com/assets/1147913/4205014/641cb0e2-383c-11e4-8140-922c151f97f8.png)