Add force-cdn-settings plugin

Set some WP SuperCache CDN options site-wide (to hard-coded constants
-- this is improvable).

See issue noblogs-composer#24 for further details.

force-cdn-settings.php

+<?php
+/*
+ * Plugin Name: A/I - Force CDN settings
+ * Description: Forces the WP SuperCache CDN settings to be the same for all sites.
+ * Version: 0.0.1
+ * Author: Autistici/Inventati
+ * Author URI: https://autistici.org
+*/
+
+function force_cdn_setting_url() {
+    return 'https://noblogs.ai-cdn.net';
+}
+add_filter('pre_option_ossdl_off_cdn_url', 'force_cdn_setting_url');
+
+function force_cdn_setting_include_dirs() {
+    return 'wp-content,wp-includes';
+}
+add_filter('pre_option_ossdl_include_dirs', 'force_cdn_setting_include_dirs');
+
+function force_cdn_setting_exclude() {
+    return '.php,blogs.dir,openpgp';
+}
+add_filter('pre_option_ossdl_exclude', 'force_cdn_setting_exclude');