Gutenberg: Add a help link directly in our block.

To assist users into finding their link for a Google Drive item, a
help link is now added below the URL field.

The help link defaults to our Github wiki page, but developers can
change the link with the 'ray_gdoc_help_url' filter to point to
another page.

See #13.

block.js

@@ -527,7 +527,13 @@
 						placeholder: i18n.__( 'Type or Paste URL. Hit Enter to submit.' ),
 						className: 'components-placeholder__input',
 						onKeyDown: this.onKeyDown
-					})
+					}),
+					el( 'div', { className: 'components-placeholder__learn-more' },
+						el( wp.components.ExternalLink, {
+							href: rayGDriveProps.helpUrl,
+							children: i18n.__( 'Find out how to find your Google Drive link' )
+						})
+					)
 				)
 			);
 		}

gdocs-shortcode.php

@@ -56,6 +56,34 @@ function ray_google_docs_gutenberg_init() {
 }
 add_action( 'init', 'ray_google_docs_gutenberg_init' );
 
+/**
+ * Enqueues inline JS for Gutenberg.
+ *
+ * Requires at least WordPress 5.2.0.
+ *
+ * @since 0.5.0
+ */
+function ray_google_docs_enqueue_block_assets() {
+	// If not WP 5.2 or if our Google Drive block is not registered, bail.
+	if ( ! function_exists( 'wp_is_xml_request' ) || ! WP_Block_Type_Registry::get_instance()->is_registered( 'ray/google-drive' ) ) {
+		return;
+	}
+
+	/**
+	 * Filter to change the default help URL for the Google Drive block.
+	 *
+	 * @since 0.5.0
+	 *
+	 * @param string $url Help URL.
+	 */
+	$help_url = apply_filters( 'ray_gdoc_help_url', 'https://github.com/cuny-academic-commons/google-docs-shortcode/wiki/Sharing-a-Google-Drive-file-and-getting-the-link' );
+
+	wp_localize_script( 'ray-gdoc-block', 'rayGDriveProps', array(
+		'helpUrl' => esc_url_raw( $help_url )
+	) );
+}
+add_action( 'enqueue_block_editor_assets', 'ray_google_docs_enqueue_block_assets', 11 );
+
 /**
  * Shortcode to embed a Google Doc.
  *