Newsletters: allow classic themes to preview or edit modals

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

Start by locating the Newsletter modal settings and the template-editor entry points for classic themes. Review how block-template support and modal preview links are handled, then confirm which proposed approach is selected; done means classic-theme customers can preview or edit the modals with clear guidance.

Written by the indexing model from the issue text.