Add dashboard panel types

Summary: Ref T3583. These will be the primary class carrying panel implementations.

Test Plan:
{F149125}

{F149126}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D8912

src/applications/dashboard/controller/PhabricatorDashboardListController.php

@@ -28,6 +28,9 @@ final class PhabricatorDashboardListController
       ->setViewer($user)
       ->addNavigationItems($nav->getMenu());
 
+    $nav->addLabel(pht('Panels'));
+    $nav->addFilter('panel/', pht('Manage Panels'));
+
     $nav->selectFilter(null);
 
     return $nav;