Use phutil_hashes_are_identical() when comparing hashes in Phabricator

Summary: See D14025. In all cases where we compare hashes, use strict, constant-time comparisons.

Test Plan: Logged in, logged out, added TOTP, ran Conduit, terminated sessions, submitted forms, changed password. Tweaked CSRF token, got rejected.

Reviewers: chad

Reviewed By: chad

Subscribers: chenxiruanhai

Differential Revision: https://secure.phabricator.com/D14026