Feature switches now have an sell-by-date

The idea of an automated expiry date has worked well for our AB testing. It asks people to think how long a feature should last in production, rather than an indefinite default. A lot of the legacy/complexity in the Guardian code base exists because, incrementally, week by week, things have been added to the project, forgotten, and never taken away again.

This will force people to periodically evaluate things by virtue of them disappearing in production and needing work to keep them alive. Think [a dead man's switch](http://en.wikipedia.org/wiki/Dead_man's_switch).

If a feature has expired, two things happen.

i. The feature is disabled (I.e. it's state is always _false_, even if it's switched on).
ii. The unit tests will break, at which point someone should remove it or seek justification as to why it should live on.

We display features expiring within 7 days on the radiator.

![expiring](https://f.cloud.github.com/assets/84996/1889679/7d894af2-7a20-11e3-8c49-8a1d06f3bff3.png)

/cc @gklopper 

![imgres](https://f.cloud.github.com/assets/84996/1880279/1e19eb6e-7956-11e3-8e5d-ce8134e7b89a.jpg)