Fix preview routes

In preview, only explicit `routes` are going to `facia` with the rest going to `applications`.

We want the same behaviour as `PROD`, with all fronts going to `facia`, and then issuing an `X-Accel-Redirect` to `applications` if it doesn't handle it.

To mimic this, we use direct calling of the `applications` controller instead of issuing an `X-Accel`.

@gklopper