Allow administrators to set a dashboard as a global default

Summary:
Ref T4883.

  - When an administrator installs a dashbord, give them the option to install it as a global default.
  - On the home page, if a user does not have a dashboard installed, check for a global default.
  - On the Admin NUX/Welcome page, check for a global dashboard.

Test Plan:
  - Installed a global dashboard, checked homepage, saw it.
  - Installed a personal dashboard over it.
  - Checked non-admin flow.
  - Checked Admin NUX page for quest completion.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4883

Differential Revision: https://secure.phabricator.com/D9670

src/applications/home/controller/PhabricatorHomeMainController.php

@@ -21,6 +21,14 @@ final class PhabricatorHomeMainController
       $user,
       $user->getPHID(),
       get_class($this->getCurrentApplication()));
+
+    if (!$dashboard) {
+      $dashboard = PhabricatorDashboardInstall::getDashboard(
+        $user,
+        PhabricatorApplicationHome::DASHBOARD_DEFAULT,
+        get_class($this->getCurrentApplication()));
+    }
+
     if ($dashboard) {
       $content = id(new PhabricatorDashboardRenderingEngine())
         ->setViewer($user)