#7727007 Email stats open rate UI

Proposed Changes

This PR adds the open rate to the email stats.

• It introduces the concept of period bound stats & all-time stats. Because of this I refactored the Redux actions and selectors a bit.
• It uses a component similar to <HighlightCard /> to render the info on top.
• It also fetched the post, so we’re able to show the title. While this is being fetched, a spinner is shown.

Testing Instructions

• Visit /stats/email/open/<site>/day/<postid>?flags=newsletter/stats. You can add the startDate parameter, to load the stats for a specific date.
• Open rate should be at the top

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?