Only display featured image UI when theme supports it too

[danielbachhuber]

Description

Ensures the "Featured Image" UI only displays when the theme supports post-thumbnails too.

Fixes #2523
Previously #6510

Types of changes

My code introduces a new ThemeSupportCheck component to inspect the theme's post-thumbnails setting. If the theme doesn't support post thumbnails, then the featured image UI doesn't render.

The code also introduces post-thumbnails=true on the site index for exposure through the REST API.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

How has this been tested?

Using Twenty Seventeen, here's the UI that normally displays:

Then, add the following code snippet to a wp-content/mu-plugins/local.php file:

add_action( 'after_setup_theme', function(){
	remove_theme_support( 'post-thumbnails' );
}, 100 );

The "Featured Image" will no longer display:

[danielbachhuber]

Worth noting: I wanted to put theme-support-check in @wordpress/components, but doing so apparently causes a dependency issue with withSelect().

[gziolo]

That’s neat idea 👍

[aduth]

Works well 👍

[aduth]

Nit (Style): My preference (and predominantly throughout the codebase) is for JSX to be placed on its own lines with parentheses if multi-lined, e.g.:

return (
	<ThemeSupportCheck supportKeys="post-thumbnails">
		<PostTypeSupportCheck { ...props } supportKeys="thumbnail" />
	</ThemeSupportCheck>
);

If it's a convention we want to adopt, we could enforce with:

https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/jsx-wrap-multilines.md

[danielbachhuber]

My preference (and predominantly throughout the codebase) is for JSX to be placed on its own lines with parentheses if multi-lined, e.g.:

Seems reasonable to me 1916026