Revamp notice appearing in AMP Enabled toggle, revamp how plugin determines whether AMP can be disabled

[westonruter]

Summary

Fixes #1864.
Fixes #2724.
FIxes #4593.

This will require heavy rewriting of https://amp-wp.org/documentation/how-the-plugin-works/amp-plugin-serving-strategies/

AMP Unavailable Notice

• When a post cannot be served as AMP, show the notice as informational rather than as a warning.
• Start the notice with AMP Unavailable to give context for what the errors are.
• When a link appears in a notice, have it open in a new window.
• In the Classic editor, the notice continues to be displayed only when clicking the "Edit" link. In Gutenberg, the notice is displayed by default but inside of a details element which is expanded to show the notices.
• If the user cannot manage_options then not AMP toggle is displayed and no notice is shown. Non-administrators will not see the toggle nor any notice if AMP is unavailable, sine they do not have the capability to take any action anyway.

Change	Gutenberg	Classic
Before		(After clicking the edit link)
After (Collapsed)		(Looks same as before)
After (Expanded)
After (Non-admin)

DB Upgrade

• Wait until the end of init to perform the DB upgrade. This ensures that themes have declared theme support and post type support which we'll then persist in the DB going forward.

Theme Support

• When add_theme_support('amp') is present, default to paired as being true as opposed to false. This only impacts the default theme_support value the first time the plugin is activated.
• The templates_supported flag now only affects the default value for the options for all_templates_supported and supported_templates (more below).

Post Types

• Eliminate the use of amp post type support to programmatically override the ability for the user to enable or disable a AMP for a given post type.
• Migrate to storing options as associative arrays: store supported_post_types option as associative array mapping the post type to a boolean. Migrate previous representation of string array.
• The first time the plugin is activated, when the amp-options option is empty: use the amp post type support to populate the default supported_post_types.

Templates

• Eliminate the use of amp theme support to programmatically override the ability for the user to control whether all templates are supported and which specific templates are supported.
• Eliminate the ability to use the amp_supportable_templates filter to programmatically override whether a template supported in AMP. If a filter tries to programmatically supply supported or immutable then _doing_it_wrong() is raised.
• Migrate to storing options as associative arrays: store supported_templates option as an associative array mapping template ID to a boolean. Migrate previous representation of string array.
• The first time the plugin is activated, when the amp-options option is empty: Use the templates_supported theme support arg to specify the default supported_templates option (make it false if the theme support arg is not 'all', and if it is an array use it to merge with the default supported_templates value.

Testing

To test, it's helpful to write out this JSON out to a file (e.g. options.json):

{"theme_support":"transitional","supported_post_types":["post"],"analytics":[],"all_templates_supported":false,"supported_templates":["is_singular"],"version":"1.5.5"}

And then load up the old plugin options via:

cat options.json | wp option set amp-options --format=json

It's also helpful to have these plugins to activate/deactivate to see how they impact the migration:

<?php
/**
 * Plugin Name: AMP Add Post Type Support for Pages
 */

add_action( 'init', function() {
	add_post_type_support( 'page', 'amp' );
} );

<?php
/**
 * Plugin Name: AMP Add Theme Support Templates Supported All
 */

add_action(
	'after_setup_theme',
	function () {
		add_theme_support( 'amp', [ 'templates_supported' => 'all' ] );
	}
);

<?php
/**
 * Plugin Name: AMP Add Theme Support Templates Supported Selectively
 */

add_action(
	'after_setup_theme',
	function () {
		add_theme_support(
			'amp',
			[
				'templates_supported' => [
					'is_search' => true,
					'is_404'    => false,
					'is_date'   => true,
				],
			]
		);
	}
);

<?php
/**
 * Plugin Name: AMP Skip Post All
 */

add_filter( 'amp_skip_post', '__return_true' );

Other

• Remove obsolete code for password-protected posts which was missed when resolving Add support for forcing AMP on protected posts #3428.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for e07656a are ready 🛎️!

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[pierlon]

Do you think AMP_Post_Meta_Box::get_status_and_errors should be refactored into 2 separate methods? The returned status is now only being used in 1 instance (AMP_Post_Meta_Box::get_amp_enabled_rest_field), and the set of errors are used twice elsewhere (AMP_Post_Meta_Box::enqueue_block_assets and AMP_Post_Meta_Box::render_status).

[westonruter]

It's being used both in AMP_Post_Meta_Box::sanitize_status() and AMP_Post_Meta_Box::render_status(), in the latter case it appears to not be used only because the usage is in the include file at the end of the method.

Nevertheless, I'm not in love with get_status_and_errors.

[westonruter]

Byebye!

[pierlon]

Functionality works as intended, only a minor visual fix I'd like to suggest.

[westonruter]

Just noticed that WordPress 5.5 removes the blue background for the notices. The margins are also a bit off, but this is out of scope of this PR. For example, the Featured Image notice:

[westonruter]

Two follow-up items I've addressed in #5042:

• AMP_Core_Theme_Sanitizer::extend_theme_support() would fail to add amp theme support if a core theme doesn't have any special theme support args. This resulted in Twenty Sixteen getting Transitional mode by default, but Twenty Twenty did not.
• When a child theme of a core theme is active, we should not default to Transitional or Standard because all bets are off as to whether it actually supports AMP. Fixed in f8c8390. We'll only be able to change that once we have the theme scan feature (Wizard Step: Theme compatibility scanning #4795).