update DEBUGKIT_JQUERY_URL

add Url helper
update DEBUGKIT_JQUERY_URL
It should be `'/debug_kit/js/jquery.js'` instead of `'/DebugKit/js/jquery.js'`. While currently it will work if the file is served by `AssetFilter`, it won't if the plugin assets are copied under app's webroot, where the folder would be `debug_kit` not `DebugKit`.