Tune cookie behaviors for 'phcid', 'phreg', etc

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