Introduce native Gutenberg controls

• Register meta
• Enqueue script
• Switch to use_block_editor_for_post_type and remove the option
• Build component
• Add sanitize callback to handle special cases
• Add auth callback to allow "private" meta to be edited
• Add padding between modal buttons
• Don't hide UI if set to 0, as there's no way to update the setting
• Switch to @wordpress/scripts
• Consider moving Gutenberg bits, and/or REST API elements, to their own class.

assets/build/gutenberg.asset.php

-<?php return array('dependencies' => array('wp-api-fetch', 'wp-components', 'wp-compose', 'wp-data', 'wp-edit-post', 'wp-element', 'wp-i18n', 'wp-plugins'), 'version' => '055b75d79581b13ad80a');
+<?php return array('dependencies' => array('wp-api-fetch', 'wp-components', 'wp-compose', 'wp-data', 'wp-edit-post', 'wp-element', 'wp-i18n', 'wp-plugins'), 'version' => '75f14aee54cc9b52ebe9');