Rss cleanup

After our `RSS` incident, this cleans up anything we did.

I have opted for explicit `/rss` routes in the `routes` files of both `facia` and `applications`. This means that the `path`/`id` should be correct in both.

The weird part is having to append `/rss` onto the `X-Accel` for `applications` as the `facia` `routes` will have swallowed it.

This depends on https://github.com/guardian/platform/pull/489

@gklopper