Fix response for home template in /pages for Coming Soon sites

[candy02058912]

Proposed Changes

• template_redirect returns a HTML response instead of a JSON response expected here
• This results in null response for https://public-api.wordpress.com/wpcom/v2/sites/:blog_id/block-editor?_envelope=1
  (notice the null values in home_template), which then prevents the Homepage template from being rendered (screenshots below)

{
	"body": {
		"is_fse_eligible": true,
		"is_fse_active": false,
		"home_template": {
			"postType": null,
			"postId": null
		}
	},
	"status": 200,
	"headers": {
		"Allow": "GET"
	}
}

Before	After

Testing Instructions

This is a fix for atomic sites in "Coming Soon" mode.

• Checkout to this branch
• Follow the steps here (PCYsg-ly5-p2) for testing editing-toolkit changes on atomic sites
• Go to /pages/:site to see if it renders the homepage
• Alternatively: check the API response for https://public-api.wordpress.com/wpcom/v2/sites/:blog_id/block-editor?_envelope=1

{
	"body": {
		"is_fse_eligible": true,
		"is_fse_active": false,
		"home_template": {
			"postType": "wp_template",
			"postId": "quadrat-white\/\/index" // the fse-supported theme you chose
		}
	},
	"status": 200,
	"headers": {
		"Allow": "GET"
	}
}

Feel free to test around the issue to see if private atomic/simple sites are functioning correctly.

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)?

Fixes #71375

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-52673

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-52673&env=jetpack

[matticbot]

This PR modifies the release build for editing-toolkit

To test your changes on WordPress.com, run install-plugin.sh editing-toolkit fix/private-site-home-template on your sandbox.

To deploy your changes after merging, see the documentation: PCYsg-mMA-p2

[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.

[bangank36]

LGTM

[arthur791004]

Looks like this way doesn't work when the Privacy of the atomic site is set to "Private" 😔