Live Preview: Coexist “Live preview” with “Demo site” #8711633

Related to #80134
#80310

Proposed Changes

Coexist “Live preview” with “Demo site” while avoiding the users’ confusion. See #80134 and pbxlJb-3Uv-p2#comment-2868 for more detailed UI conversations.

This PR covers the following scenarios;

on the Theme Card three dots menu
For themes that have Live Preview functionality, show Live preview
For other themes (that has demo site URL), show Demo site

on Theme Detail view
Show the Preview demo site button by hovering over the right side column preview
Show Demo site as a button for themes that do NOT have the Live Preview functionality

Testing Instructions

Theme Card three dots menu

• Go to the Theme Showcase page
• Find the “Live preview” option on the Theme Card three dots menu
  • For themes that have Live Preview functionality, show Live preview
  • For other themes, show Demo site

Theme Detail page

• Go to the Theme Detail page
• Find the “Live preview” button
  • Show the Preview demo site button by hovering over the right side column preview
  • Show Demo site as a button for themes that do NOT have the Live Preview functionality

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)?