Show a "Logout?" dialog when the user goes to /logout/

Summary: Currently, the logout link is this awkward form in the footer since we have to CSRF it. Enable a GET + dialog + confirm workflow instead so the logout link can just be a link instead of this weird mess.

Test Plan: Went to /logout/, logged out.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3066

src/applications/auth/controller/PhabricatorLogoutController.php

@@ -58,6 +58,17 @@ final class PhabricatorLogoutController
         ->setURI('/login/');
     }
 
+    if ($user->getPHID()) {
+      $dialog = id(new AphrontDialogView())
+        ->setUser($user)
+        ->setTitle('Log out of Phabricator?')
+        ->appendChild('<p>Are you sure you want to log out?</p>')
+        ->addSubmitButton('Log Out')
+        ->addCancelButton('/');
+
+      return id(new AphrontDialogResponse())->setDialog($dialog);
+    }
+
     return id(new AphrontRedirectResponse())->setURI('/');
   }