Store winning variations for later applications

For each test type when a winner is found we should log the variants data in the following shape:

```js
{
"value": "string", // mixed - value of the variant
"score": 2.0, // percentage change in conversion compared to control
}
```

We could later use this information for a few applications:

- recommending alternatives based on higher performers
- warning if some data is similar to a value that didn't perform well