Disable app banner on FirefoxOS App

Due to some great work by @jamesgorrie we now have the site wrapped in a FirefoxOS app, which is great and can be found in the market place [here](https://marketplace.firefox.com/app/guardian)

Unfortunately, our native app upsell banner is kicking in inside the FirefoxOS app trying to get the user to install the Android app. This is not good and is leading to tweets such as this: https://twitter.com/firt/status/526786343805136896

![](https://pbs.twimg.com/media/B0-FYr1IMAA_Cxh.png:small)

This PR refactors our smart up logic to exclude OS apps, and abstracts the UA detection into `detect.js` where it belongs.

Thanks to @kaelig for pointing this out to us.