Switches now have an sell-by-date

The idea of an automated expiry date has worked well for our AB testing. It forces people
to think how long a feature should last in production, rather than the indefinate default.

If a feature has expired two things happen.

i. The feature is disabled.
ii. The tests will break, at which point someone should remove it or seek justification as to why it should live on.

The sell-by-date is optional at the moment (it default to 'the future'), but we'll make it permanent once the concept is understood by everyone.
Merge branch 'master' of https://github.com/guardian/frontend into sell_by_date

Conflicts:
	common/app/conf/switches.scala
Formatting
Merge branch 'master' of https://github.com/guardian/frontend into sell_by_date
Add features expiring within 7 days to the radiator
rename AlwaysOnSwitch -> NeverExpiresSwitch
All features now mandate an expiry date
The future is 2100
Switchboard displays expiry date
Merge pull request #2711 from guardian/sell_by_date

Feature switches now have an sell-by-date