Newsletters: allow classic themes to preview or edit modals

[simison]

We allow customers to enable modals and few other things in classic themes, but since preview/editing happens in the template editor, we don't allow them to edit them:

Block themes:

Classic themes:

We could however:

• Explore allowing edit links if classic theme declares add_theme_support( 'block-template-parts' );
• Add copy indicating that with more modern theme they could preview/edit the modals. Perhaps in a popover.
• Add preview links which are just static links to the site, but with some URL attribute forcing the modals to show up even if they're disabled, e.g. ?jetpack-preview-welcome-overlay etc

Came up in p1718967449951009-slack-C052XEUUBL4