Editor: Allow composing block editor posts on WordPress.com.

[kwight]

This PR allows WordPress.com users to access the block editor for Jetpack sites on Calypso via an iframe.

See phase 2 notes at p3fqKv-6Hh-p2.

Testing Instructions

• Sandbox the API and apply D27129-code to it.
• Load Calypso either locally or by calypso.live (this feature is only available behind the jetpack/gutenframe flag on testing environments, being wpcalypso, horizon, and development).
• Directly load the URL /block-editor/post/:yourJetpackDomain.
• Verify the block editor iframe does not load (you will see a gray screen with a sameorigin error in the console).
• Load this branch.
• Disconnect and reconnect your site to auto-activate the new module.
• If that doesn't work, activate it from the oldskool modules page at wp-admin/admin.php?page=jetpack_modules (click "WordPress.com Block Editor Iframe" towards the bottom).
• Directly load the URL /block-editor/post/:yourJetpackDomain again.
• Verify the iframed block editor loads as expected.

Proposed changelog entry for your changes:

• Block Editor: WordPress.com users can now compose Block Editor posts for their Jetpack sites without leaving WordPress.com.

[jetpackbot]

	Warnings
⚠️	"Testing instructions" are missing for this PR. Please add some
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes
⚠️	Consider adding new PHP files to PHPCS whitelist for automatic linting: modules/wordpress-com-block-editor-iframe.php

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

Generated by 🚫 dangerJS against 0bfded6

[kwight]

Noting that this still in progress, but will proceed attempting to use Jetpack_Signature for verification instead of core nonces. Next steps are to determine how best to replace the modules/wordpress-com-compose.phpL31 nonce check with a new method based on Jetpack_Signature, and then update the server-side in D24388-code. cc/ @obenland

[kraftbj]

Question: Is this something you're aiming for 7.2 (April release) or just whenever it is ready?

[obenland]

@kraftbj There's no ETA yet, this is more experimental

[kwight]

Noting that I stumbled across #7623 today; so if we go ahead with this, we'll need to consider overlapping concerns.

[kraftbj]

Could you update the testing instructions? I can't find the setting and the linked wp.com phab is closed. Thank you!

[mdawaffe]

What happens if the user on the Jetpack site has the Classic Editor active? (If it's a problem, and needs fixing in Jetpack, it probably deserves a separate PR.)

[obenland]

What happens if the user on the Jetpack site has the Classic Editor active? (If it's a problem, and needs fixing in Jetpack, it probably deserves a separate PR.)

Yes, that is something that's being explored in Automattic/wp-calypso#32047

[mdawaffe]

💥

[codebykat]

Added to the 7.3 Changelog in 7514c5f