Starter Page Templates: Use template/demo endpoint for the preview

[andrewserong]

This is an alternative to #49101 for dealing with attempting to resolve some of the preview issues with the starter page templates. It attempts to resolve it by using the template/demo endpoint for the previews, however there are some drawbacks with this approach:

• The contact forms still aren't rendering correctly (raised in Add New Page Screen: Contact Form not rendering correctly in preview #48889)
• The preview will render with the user's selected theme, however won't pick up any colour changes they've made
• If the user has changed their fonts it won't pick them up either
• Lists of blog posts will not render with the user's content (however there's an update to use better looking placeholder content in D56259-code)

Screenshot

Note in this screenshot, it doesn't address the issue in #48889, as it's instead showing a different problem with the preview:

Changes proposed in this Pull Request

• Try out replacing the main preview for starter page templates with the template/demo endpoint

Testing instructions

• Apply the diff associated with this PR and sandbox your test site
• Go to create a new page, and preview the different page layouts
• Change theme and continue previewing

Related to #

[matticbot]

Test live: https://calypso.live/?branch=update/use-template-demo-endpoint-for-spt-preview

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

Caution: This PR affects files in the Editing Toolkit Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D56263-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing the Editing Toolkit Plugin for more info: PCYsg-ly5-p2

[andrewserong]

Closing as this approach is not viable.