Roles/Capabilities: Add meta-caps for comment, term, and user meta.

Additionally, use these meta-caps in the REST API endpoints.

Previously, register_meta()'s auth_callback had no effect for non-post meta. This introduces `{add,edit,delete}_{comment,term,user}_meta` meta-caps to match the existing post meta capabilities. These are currently only used in the REST API.

Props tharsheblows, boonebgorges.
Fixes #38303, fixes #38412.

Built from https://develop.svn.wordpress.org/trunk@39179


git-svn-id: http://core.svn.wordpress.org/trunk@39119 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php

@@ -631,7 +631,7 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 			if ( ! $change ) {
 				return new WP_Error( 'rest_comment_failed_edit', __( 'Updating comment status failed.' ), array( 'status' => 500 ) );
 			}
-		} else {
+		} elseif ( ! empty( $prepared_args ) ) {
 			if ( is_wp_error( $prepared_args ) ) {
 				return $prepared_args;
 			}