#7687064 Stats: Polish post detail tables with existing stats table styles

Proposed Changes

• Polish and reuse the existing stats table and control styles on the table section of the post detail page.
• Gate the featured changes on PostMonths and PostWeeks by the flag stats/enhance-post-detail.
• Extract the shared stats table styles as a separate file.

Testing Instructions

For a11n, if you have no idea about the testing data on the tables, please refer to p1671729060450249/1671728562.298969-slack-C0438NHCLSY.

• Spin up this change with the Calypso Live link.
• Navigate to the Stats Post Detail page (/stats/post/${post-id}/${site-id}).
• Ensure the post detail page works as previously.
• Append the feature flag ?flags=stats/enhance-post-detail to the URL.
• Ensure the post detail page table styles are polished in line with the design.

Months and years table

Before	After

Average per day table

Before	After

Recent weeks table

Before	After

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)?

Related to #70671