Add minimum sample size calculator & test period helper to A/B test UI

Getting good results from an A/B test is much more likely with a suitable minimum sample size. This should be based on the calculator linked below but also based on any existing / current data available if possible, unfortunately for a new test there might not be existing conversion rate data but we could explore adding a field to enter that.

What we could do is to help determine the minimum sample size while the test is running by starting with a high value then once we have a statistically significant conversion rate for the original variant we can determine the sample size needed per variant and keep the test running until that target is reached.

https://www.evanmiller.org/ab-testing/sample-size.html