WPCOM merge: list post type taxonomies endpoint (Automattic#9428)

* REST API: CPT: Preserve custom taxonomy registrations when localizing

This revision seeks to resolve an issue where localized requests for taxonomies for themes where custom taxonomy associations exist will not preserve those associations.

For example, themes where "featured-content" is supported (e.g. Sketch) register the "post_tag" taxonomy for "page" types. If a request is issued with locale specified for this site to retrieve page taxonomies, previously the "post_tag" taxonomy was not returned.

The fix here is to track taxonomies for the requested post type before calling this method, then reapplying them after create_initial_taxonomies is called.

We need to call create_initial_taxonomies because it runs very early in the page lifecycle, before localization has had a chance to take effect.

See: Automattic/wp-calypso#6934 (comment)
Differential Revision: D3465

Merges r146417-wpcom.

* Moved post type taxonomies lister endpoint instantiation to the file that defines it.

Details: https://[private link]

Merges r161691-wpcom.

json-endpoints/class.wpcom-json-api-list-post-type-taxonomies-endpoint.php

@@ -39,12 +39,7 @@ function callback( $path = '', $blog_id = 0, $post_type = 'post' ) {
 			$this->load_theme_functions();
 		}
 
-		/** This filter is documented in jetpack/json-endpoints/class.wpcom-json-api-list-post-types-endpoint.php */
-		if ( apply_filters( 'rest_api_localize_response', false ) ) {
-			// API localization occurs after the initial taxonomies have been
-			// registered, so re-register if localizing response
-			create_initial_taxonomies();
-		}
+		$this->localize_initial_taxonomies( $post_type );
 
 		$args = $this->query_args();
 
@@ -79,4 +74,25 @@ function callback( $path = '', $blog_id = 0, $post_type = 'post' ) {
 			'taxonomies' => $formatted_taxonomy_objects,
 		);
 	}
+
+	protected function localize_initial_taxonomies( $post_type ) {
+		/** This filter is documented in jetpack/json-endpoints/class.wpcom-json-api-list-post-types-endpoint.php */
+		if ( ! apply_filters( 'rest_api_localize_response', false ) ) {
+			return;
+		}
+
+		// Since recreating initial taxonomies will restore the default post
+		// types to which they are associated, save post type's taxonomies in
+		// case it was customized via `register_taxonomy_for_object_type`
+		$post_type_taxonomies = get_object_taxonomies( $post_type );
+
+		// API localization occurs after the initial taxonomies have been
+		// registered, so re-register if localizing response
+		create_initial_taxonomies();
+
+		// Restore registered taxonomies for post type
+		foreach ( $post_type_taxonomies as $taxonomy ) {
+			register_taxonomy_for_object_type( $taxonomy, $post_type );
+		}
+	}
 }