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

src/applications/auth/controller/PhabricatorAuthStartController.php

@@ -30,6 +30,7 @@ final class PhabricatorAuthStartController
     // it and warn the user they may need to nuke their cookies.
 
     $session_token = $request->getCookie(PhabricatorCookies::COOKIE_SESSION);
+
     if (strlen($session_token)) {
       $kind = PhabricatorAuthSessionEngine::getSessionKindFromToken(
         $session_token);
@@ -87,10 +88,7 @@ final class PhabricatorAuthStartController
 
     if (!$request->isFormPost()) {
       PhabricatorCookies::setNextURICookie($request, $next_uri);
-
-      $request->setCookie(
-        PhabricatorCookies::COOKIE_CLIENTID,
-        Filesystem::readRandomCharacters(16));
+      PhabricatorCookies::setClientIDCookie($request);
     }
 
     $not_buttons = array();