Http switches

This change will allow us to turn switches on/off in non-production environments (ie CODE) and iterate on a switched off feature (ie during development) without affecting what the rest of the team sees being displayed.
Helpful when cross-functional team members work together on a feature.

This override happens only for switches explicitly wrapped in HttpSwitch (ie HttpSwitch(AudienceScienceSwitch).isSwitchedOn instead of AudienceScienceSwitch.isSwitchedOn) and within the HTTP scope.

![](http://media0.giphy.com/media/qiGjCzdYo6Qne/giphy.gif)
This should probably mixin `com.gu.management.Switchable`. That way it is just a Switch like everything else