Use correct viewer for debug.stop-on-redirect

Summary:
See <https://github.com/facebook/phabricator/pull/563>.

I think this secondary construction of a `$user` is very old, and predates subsequent changes which cause a proper user to construct earlier, so using the user on the `$request` should (I think) always work. I couldn't immediately find any cases where it does not.

Test Plan: With `debug.stop-on-redirect` set, hit various redirects, like jump-naving to T1. Got a proper stop dialog.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8718