Replace "web" and "conduit" magic session strings with constants

Summary: Ref T4310. Ref T3720. We use bare strings to refer to session types in several places right now; use constants instead.

Test Plan: grep; logged out; logged in; ran Conduit commands.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4310, T3720

Differential Revision: https://secure.phabricator.com/D7963

src/applications/auth/controller/PhabricatorAuthController.php

@@ -65,7 +65,7 @@ abstract class PhabricatorAuthController extends PhabricatorController {
   protected function loginUser(PhabricatorUser $user) {
 
     $response = $this->buildLoginValidateResponse($user);
-    $session_type = 'web';
+    $session_type = PhabricatorAuthSession::TYPE_WEB;
 
     $event_type = PhabricatorEventType::TYPE_AUTH_WILLLOGINUSER;
     $event_data = array(