Summary: Fixes T3471. Specific issues: - Add the ability to set a temporary cookie (expires when the browser closes). - We overwrote 'phcid' on every page load. This creates some issues with browser extensions. Instead, only write it if isn't set. To counterbalance this, make it temporary. - Make the 'next_uri' cookie temporary. - Make the 'phreg' cookie temporary. - Fix an issue where deleted cookies would persist after 302 (?) in some cases (this is/was 100% for me locally). Test Plan: - Closed my browser, reopned it, verified temporary cookies were gone. - Logged in, authed, linked, logged out. Reviewers: btrahan Reviewed By: btrahan Subscribers: epriestley Maniphest Tasks: T3471 Differential Revision: https://secure.phabricator.com/D8537