Consolidate use of magical cookie name strings

Summary: Ref T4339. We have more magical cookie names than we should, move them all to a central location.

Test Plan: Registered, logged in, linked account, logged out. See inlines.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4339

Differential Revision: https://secure.phabricator.com/D8041

src/applications/auth/controller/PhabricatorAuthLinkController.php

@@ -79,7 +79,10 @@ final class PhabricatorAuthLinkController
 
     $panel_uri = '/settings/panel/external/';
 
-    $request->setCookie('phcid', Filesystem::readRandomCharacters(16));
+    $request->setCookie(
+      PhabricatorCookies::COOKIE_CLIENTID,
+      Filesystem::readRandomCharacters(16));
+
     switch ($this->action) {
       case 'link':
         $form = $provider->buildLinkForm($this);