Fix several issues with "Logout"

Summary:
Ref T1536.

  - When we render a dialog on a page by itself, put it on a dust background.
  - Currently, we render "Logout" in two different places. Stop doing that.
  - Make sure the surviving one has workflow so we get a modal ajax dialog if possible.

Test Plan: {F46731}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6226

src/applications/base/controller/PhabricatorController.php

@@ -224,6 +224,7 @@ abstract class PhabricatorController extends AphrontController {
         $view->appendChild(hsprintf(
           '<div style="padding: 2em 0;">%s</div>',
           $response->buildResponseString()));
+        $view->setDust(true);
         $response = new AphrontWebpageResponse();
         $response->setContent($view->render());
         return $response;