R1:e0aa33c46b0c

Make test for setting "next" cookie more general

Summary:
Ref T6870. Since it does not make sense to redirect the user to the login form after they log in, we try not to set the login form as the `next` cookie.

However, the current check is hard-coded to `/auth/start/`, and the form can also be served at `/login/`. This has no real effect on normal users, but did make debugging T6870 confusing.

Instead of using a hard-coded path check, test if the controller was delegated to. If it was, store the URI.