Gutenberg: add loading blocks (#10008)

Meant for easy testing of blocks during the unstable
phase of developing Jetpack's Gutenberg extensions.

- Loading blocks is disabled by default. To enable:
    ```php
    add_filter( 'jetpack_gutenberg', '__return_true', 10 );
    ```

- When enabled, loads blocks from WordPress.com CDN. 
    To load files locally instead:
    ```php
    add_filter( 'jetpack_gutenberg_cdn', '__return_false', 10 );
    ```

- To adjust CDN cache-busting:
    ```php
    add_filter( 'jetpack_gutenberg_cdn_cache_buster', function( $version ) {
        return time();
    }, 10, 1 );
    ```

Note that you will need to build block files yourself when loaded locally:
- `_inc/blocks/jetpack-editor.css`
- `_inc/blocks/jetpack-editor.js`

.gitignore

@@ -33,6 +33,7 @@ yarn-error.log
 # From here on, we only list files that are generated by a build process, ignored in git, but should be present in SVN.
 #
 ## Things we will need in release branches
+/_inc/blocks
 /_inc/build
 /modules/**/*.min.css
 /modules/**/*-rtl.css

class.jetpack.php

@@ -526,6 +526,9 @@ private function __construct() {
 			Jetpack_Network::init();
 		}
 
+		// Load Gutenberg editor blocks
+		add_action( 'init', array( $this, 'load_jetpack_gutenberg' ) );
+
 		add_action( 'set_user_role', array( $this, 'maybe_clear_other_linked_admins_transient' ), 10, 3 );
 
 		// Unlink user before deleting the user from .com
@@ -7258,4 +7261,107 @@ public static function handle_post_authorization_actions( $activate_sso = false,
 
 		Jetpack::state( 'message', 'authorized' );
 	}
+
+	/**
+	 * Check if Gutenberg editor is available
+	 *
+	 * @since 6.5.0
+	 *
+	 * @return bool
+	 */
+	public static function is_gutenberg_available() {
+		return function_exists( 'register_block_type' );
+	}
+
+	/**
+	 * Load Gutenberg editor blocks.
+	 *
+	 * This section meant for unstable phase of developing Jetpack's
+	 * Gutenberg extensions. If still around after Sep. 15, 2018 then
+	 * please file an issue to remove it; if nobody responds within one
+	 * week then please delete the code.
+	 *
+	 *
+	 * Loading blocks is disabled by default and enabled via filter:
+	 *   add_filter( 'jetpack_gutenberg', '__return_true', 10 );
+	 *
+	 * When enabled, blocks are loaded from CDN by default. To load locally instead:
+	 *   add_filter( 'jetpack_gutenberg_cdn', '__return_false', 10 );
+	 *
+	 * Note that when loaded locally, you need to build the files yourself:
+	 * - _inc/blocks/jetpack-editor.js
+	 * - _inc/blocks/jetpack-editor.css
+	 *
+	 * CDN cache is busted once a day or when Jetpack version changes. To customize it:
+	 *   add_filter( 'jetpack_gutenberg_cdn_cache_buster', function( $version ) { return time(); }, 10, 1 );
+	 *
+	 * @since 6.5.0
+	 *
+	 * @return void
+	 */
+	public static function load_jetpack_gutenberg() {
+		/**
+		 * Filter to turn on loading Gutenberg blocks
+		 *
+		 * @since 6.5.0
+		 *
+		 * @param bool false Whether to load Gutenberg blocks
+		 */
+		if ( ! Jetpack::is_gutenberg_available() || ! apply_filters( 'jetpack_gutenberg', false ) ) {
+			return;
+		}
+
+		/**
+		 * Filter to turn off serving blocks via CDN
+		 *
+		 * @since 6.5.0
+		 *
+		 * @param bool true Whether to load Gutenberg blocks from CDN
+		 */
+		if ( apply_filters( 'jetpack_gutenberg_cdn', true ) ) {
+			$editor_script = 'https://s0.wp.com/wp-content/mu-plugins/jetpack/_inc/blocks/jetpack-editor.js';
+			$editor_style = 'https://s0.wp.com/wp-content/mu-plugins/jetpack/_inc/blocks/jetpack-editor.css';
+
+			/**
+			 * Filter to modify cache busting for Gutenberg block assets loaded from CDN
+			 *
+			 * @since 6.5.0
+			 *
+			 * @param string
+			 */
+			$version = apply_filters( 'jetpack_gutenberg_cdn_cache_buster', sprintf( '%s-%s', gmdate( 'd-m-Y' ), JETPACK__VERSION ) );
+		} else {
+			$editor_script = plugins_url( '_inc/blocks/jetpack-editor.js', JETPACK__PLUGIN_FILE );
+			$editor_style = plugins_url( '_inc/blocks/jetpack-editor.css', JETPACK__PLUGIN_FILE );
+			$version = Jetpack::is_development_version() ? filemtime( JETPACK__PLUGIN_DIR . '_inc/blocks/jetpack-editor.js' ) : JETPACK__VERSION;
+		}
+
+		wp_register_script(
+			'jetpack-blocks-editor',
+			$editor_script,
+			array(
+				'wp-blocks',
+				'wp-components',
+				'wp-compose',
+				'wp-data',
+				'wp-editor',
+				'wp-element',
+				'wp-i18n',
+				'wp-plugins',
+			),
+			$version
+		);
+
+		wp_register_style(
+			'jetpack-blocks-editor',
+			$editor_style,
+			array(),
+			$version
+		);
+
+		register_block_type( 'jetpack/blocks', array(
+				'editor_script' => 'jetpack-blocks-editor',
+				'editor_style'  => 'jetpack-blocks-editor',
+		) );
+	}
 }