Add multivariate test framework

This is a client-side multivariate test framework for A/B testing stuff. It uses Google Analytics page level custom variables which can be used to segment users in different cohorts. See the comment at the top of `multivariate-test.js` for more info on how it works.

This has been initially built for this story:

https://www.pivotaltracker.com/s/projects/367813/stories/61474462

@bradleywright knows more about how multivariate testing should be working in the future, and comments as to how this helps towards that goal would be helpful. In particular, one idea is to eventually allow editors to test content themselves by giving them two different text boxes. This may be useful for that – the backend could initialise `MultivariateTest` with the different bits of content.