Admin+Config: Add toggle that disables Cloudflare cache

It's now possible to disable the Cloudflare cache during development,
so the hacky workaround with the random number in the CSS/JS URLs is no
longer needed.

Development mode is automatically disabled by Cloudflare after 4 hours.

config.php.example

@@ -14,4 +14,8 @@ return [
 	'MAIL_NAME' => '',
 	'MAIL_USERNAME' => '',
 	'MAIL_PASSWORD' => '',
+
+	'DEVELOPMENT_MODE' => '',
+	'DEVELOPMENT_MODE_TOKEN' => '',
+	'DEVELOPMENT_MODE_ZONE' => '',
 ];