Security, Site Health: Improve accuracy in messaging about HTTPS support.

Following up on [49904], this changeset focuses mainly on improving the guidance about the current state of HTTPS in Site Health.

* Correct the existing copy to indicate that both the Site Address and the WordPress Address need to be changed to fully switch to HTTPS.
* Link to the respective input fields via anchor links rather than to the overall General Settings screen.
* Show different copy if the site is using HTTPS for the WordPress Address (for example to have only the administration panel in HTTPS), but not for the Site Address.
* Inform the user about potential problems even when the site is already using HTTPS, for example if the SSL certificate was no longer valid.
* Always rely on fresh information for determining HTTPS support issues in Site Health, and therefore change the `https_status` test to become asynchronous.
* Rename the new private `wp_is_owned_html_output()` function to a more appropriate `wp_is_local_html_output()`.

Props adamsilverstein, flixos90, johnjamesjacoby, timothyblynjacobs.
See #47577.


git-svn-id: https://develop.svn.wordpress.org/trunk@50072 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-site-health-controller.php

@@ -85,6 +85,25 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 			)
 		);
 
+		register_rest_route(
+			$this->namespace,
+			sprintf(
+				'/%s/%s',
+				$this->rest_base,
+				'https-status'
+			),
+			array(
+				array(
+					'methods'             => 'GET',
+					'callback'            => array( $this, 'test_https_status' ),
+					'permission_callback' => function () {
+						return $this->validate_request_permission( 'https_status' );
+					},
+				),
+				'schema' => array( $this, 'get_public_item_schema' ),
+			)
+		);
+
 		register_rest_route(
 			$this->namespace,
 			sprintf(
@@ -199,6 +218,18 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 		return $this->site_health->get_test_loopback_requests();
 	}
 
+	/**
+	 * Checks that the site's frontend can be accessed over HTTPS.
+	 *
+	 * @since 5.7.0
+	 *
+	 * @return array
+	 */
+	public function test_https_status() {
+		$this->load_admin_textdomain();
+		return $this->site_health->get_test_https_status();
+	}
+
 	/**
 	 * Checks that the authorization header is valid.
 	 *