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.

.gitattributes

@@ -4,10 +4,12 @@
 /.gitattributes export-ignore
 /.gitignore export-ignore
 /.gitlab-ci.yml export-ignore
+/.nvmrc export-ignore
 /.travis.yml export-ignore
 /.DS_Store export-ignore
 /.wordpress-org export-ignore
 /Thumbs.db export-ignore
+/assets/src export-ignore
 /behat.yml export-ignore
 /bitbucket-pipelines.yml export-ignore
 /bin export-ignore
@@ -31,6 +33,7 @@
 /tests export-ignore
 /vendor export-ignore
 /node_modules export-ignore
+/webpack.config.js export-ignore
 /*.sql export-ignore
 /*.tar.gz export-ignore
 /*.zip export-ignore