Added meta tag to upsell iPhone app

It is behind a switch (off by default)

Will be a simple matter to enable it and then see if we get higher registrations.

Only on UK edition (US has different app)

@kungpochicken I think we should at least give it a run for a day and see what happens...

![2013-06-21 16 27 51](https://f.cloud.github.com/assets/76709/687955/c7ea7da0-da89-11e2-8c63-5e47ca116956.png)