fix(trend experiment): fix credible interval delta

[jurajmajerik]

Problem

Forgot to divide by the control mean to get the correct credible interval delta.

I didn't catch it because, by coincidence, the result still made sense in my local example. A user pointed it out here: https://posthog.slack.com/archives/C06KJQ2RH1A/p1724761173381449

How did you test this code?

👀

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[jurajmajerik]

Added a comment + smuggling two more changes, as users reported these as confusing:

• the summary now mentions the win percentage for both Trends and Funnels, making it consistent
• the credible interval for Funnel is now also expressed w.r.t. %Delta, same like Trends

[posthog-bot]

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