Show A/B Test Results

This is a first attempt at showing A/B test results, specifically in this case for the [Swipe test](https://github.com/guardian/frontend/blob/master/docs/experiments/swipe-ctas.md).

It adds a new graph to /analytics/kpis showing average page views per user session per day per variant.

Next steps:
* Create new endpoint specifically for A/B test results
* Distinguish between behaviour of loyal visitors (ie repeat visitors, in some sense) and new visitors
* Show dwell/engagement time per visitor per variant