Asset CDN: Fix json translation loading (#10982)

When the static asset CDN of Jetpack is loaded, the translations for Gutenberg don'tt load properly because the filename is generated for a wrong path.

Also fixes Automattic/wp-calypso#29309

This has been extensively discussed in https://core.trac.wordpress.org/ticket/45528

#### Changes proposed in this Pull Request:

* This implements the filter that has been introduced into Core.

#### Testing instructions:

Prequisit: WordPress 5.0.2(alpha) or trunk

1. Set your WordPress to another language than English.
2. Activate Jetpack Static Asset CDN.
3. Potentially add a filter to your WordPress if it's running trunk (see #10102)
4. Create a new post to open Gutenberg and observe broken translations.
5. Activate the patch: Translations should now load after a reload.

#### Proposed changelog entry for your changes:

* Fixes loading of the Block Editor UI translations when Static Asset CDN is used (requires WordPress 5.0.2)

modules/photon-cdn.php

@@ -30,6 +30,7 @@ public static function go() {
 		add_action( 'admin_print_scripts', array( __CLASS__, 'cdnize_assets' ) );
 		add_action( 'admin_print_styles', array( __CLASS__, 'cdnize_assets' ) );
 		add_action( 'wp_footer', array( __CLASS__, 'cdnize_assets' ) );
+		add_filter( 'load_script_textdomain_relative_path', array( __CLASS__, 'fix_script_relative_path' ), 10, 2 );
 	}
 
 	/**
@@ -83,6 +84,17 @@ public static function cdnize_assets() {
 		}
 	}
 
+	/**
+	 * Ensure use of the correct relative path when determining the JavaScript file names.
+	 *
+	 * @param string $relative The relative path of the script. False if it could not be determined.
+	 * @param string $src      The full source url of the script.
+	 * @return string The expected relative path for the CDN-ed URL.
+	 */
+	public static function fix_script_relative_path( $relative, $src ) {
+		return substr( $src, 1 + strpos( $src, '/wp-includes/' ) );
+	}
+
 	/**
 	 * Sets up CDN URLs for supported plugin assets.
 	 *