Add the site reading options to the index

[Mamaduka]

PR exposes the site reading settings via the REST index, which allows low-capability users to preview and change templates.

Until #48885 is resolved, the only option for exposing readonly settings is the REST API index.

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/69106.

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/69106.
Trac ticket: https://core.trac.wordpress.org/ticket/63023

[spacedmonkey]

@Mamaduka Can you review ​https://github.com/WordPress/wordpress-develop/pull/8380? If so then we can just commit that.

Create the Trac ticket and update unit test files.

I don't think it should block merge. But yeah could be a cool include or follow up.

Let's follow-up on this.

[Mamaduka]

@spacedmonkey, I had a quick look today, and it looks really good. I'll try to do a proper review/testing this week.

Unfortunately, I think we still have to commit to this change. The JS code needs to be compatible with the last two versions of WordPress. Low-capability users won't be able to load the editor without reading settings.

[Mamaduka]

In 59880:

REST API: Add the site reading options to the index.

Exposes page_for_posts, page_on_front and show_on_front reading settings via REST API index.

Props mamaduka, audrasjb, spacedmonkey, timothyblynjacobs.
Fixes #63023.

Committed https://core.trac.wordpress.org/changeset/59880.