Add internal redirects to RSS server

All RSS endpoints are sent to Facia, as at the time of serving we do not know whether they are fronts (in which case Facia serves the RSS) or index pages (in which case historically Applications has served the RSS, but now the RSS server will).

This sends appropriate internal redirects for RSS pages. I've also added a switch so that we can roll this back if there's any issues.
would `Future.successful(...)` be better?