Notes: Add email notification

[t-hamano]

• Part of Notes: Backports from core #73141
• Backport these core changesets to Gutenberg:
  • https://core.trac.wordpress.org/changeset/61179
  • https://core.trac.wordpress.org/changeset/61199

What?

This PR will add email notifications for notes to the Gutenberg plugin, which was implemented in core.

Why?

Backward compatibility.

How?

This PR is broadly divided into the following three hooks:

• gutenberg_register_wp_notes_notify_setting: Registers a note notification setting to the Discussion screen.
• gutenberg_new_comment_via_rest_notify_postauthor: Send an email when a comment is inserted via the REST API
• gutenberg_filter_note_notification_text, gutenberg_filter_note_notification_subject: Filter an email subject and content

Testing Instructions

Note: The email notification feature has been implemented in the latest WordPress core, so don't forget to downgrade the WordPress version to 6.8 and test it.

To test this PR, you'll need to set up an SMTP server in your wp-env environment. One way to do this is to temporarily set up Gmail's SMTP server:

// Setup Gmail SMTP.
add_action( 'phpmailer_init',function( $phpmailer ) {
	$phpmailer->isSMTP();
	$phpmailer->Host       = 'smtp.gmail.com';
	$phpmailer->SMTPAuth   = true;
	$phpmailer->Port       = 587;
	$phpmailer->SMTPSecure = 'tls';
	$phpmailer->FromName   = get_bloginfo( 'name' );

	// Your Gmail address.
	$phpmailer->Username   = 'example@gmail.com';
	// Your Gmail address.
	$phpmailer->From       = 'example@gmail.com';
	// Your Gmail "App" password.
	// https://support.google.com/mail/answer/185833?hl=en
	$phpmailer->Password   = 'xxxx xxxx xxxx xxxx';

	// Send all emails to the fixed address.
	$phpmailer->clearAddresses();
	$phpmailer->clearCCs();
	$phpmailer->clearBCCs();
	$phpmailer->clearReplyTos();
	// Your Gmail address.
	$phpmailer->addAddress( 'example@gmail.com' );
} );

// Set the from email address.
add_filter( 'wp_mail_from', function( $email ) {
	// Your Gmail address.
	return 'example@gmail.com';
} );

// Log failed emails. If you encounter an error, please check the log.
add_action( 'wp_mail_failed', function ( $error ) {
	error_log( print_r( $error->get_error_message(), true ) );
} );

2. Create a post.
3. Log in with a different user account and add a note to that post.
4. An email should be sent.

[t-hamano]

Unfortunately, the UI is different from the core, as it is not possible to inject new settings into existing sections.

Core

Gutenberg

[Mamaduka]

We only need to register this setting for WP < 6.9. I think it's currently registered for all versions.

[t-hamano]

I thought we could check if the settings were already registered via get_registered_settings, but apparently that's not the case. Fixed in 1a34a21.

[github-actions]

Flaky tests detected in 667c0c4.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/20050507215
📝 Reported issues:

• [Flaky Test] All navigation links should default to the body color and submenus and mobile overlay should default to a white background with black text #55304 in /test/e2e/specs/editor/blocks/navigation-colors.spec.js

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

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[adamsilverstein]

Nice work! left some small nitpick feedback. Also, I only reviewed the code, it would be good to have some manual testing which I did not do.

[Mamaduka]

Tested using WP Mail Debugger and this is working as expected.

[t-hamano]

Thanks both of you for the reviews!