The plug the internal catch-all route points at.
Its job is to undo the rewrite before the controller sees it. Matching a
database-backed URL means temporarily replacing conn.path_info with an
internal one so that Phoenix's compiled router has something to match; if
that replacement were still in place inside the controller, every canonical
URL it built would name a path no client ever requested.
So this sits between the route and the controller: the pipeline has already
run, the path goes back to what the client actually sent, and the controller
is invoked with a conn that looks exactly as it would have if the route
had been declared in the first place.