Add "High Security" mode to support multi-factor auth

Summary:
Ref T4398. This is roughly a "sudo" mode, like GitHub has for accessing SSH keys, or Facebook has for managing credit cards. GitHub actually calls theirs "sudo" mode, but I think that's too technical for big parts of our audience. I've gone with "high security mode".

This doesn't actually get exposed in the UI yet (and we don't have any meaningful auth factors to prompt the user for) but the workflow works overall. I'll go through it in a comment, since I need to arrange some screenshots.

Test Plan: See guided walkthrough.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8851

src/aphront/configuration/AphrontDefaultApplicationConfiguration.php

@@ -123,6 +123,49 @@ class AphrontDefaultApplicationConfiguration
       return $response;
     }
 
+    if ($ex instanceof PhabricatorAuthHighSecurityRequiredException) {
+
+      $form = id(new PhabricatorAuthSessionEngine())->renderHighSecurityForm(
+        $user,
+        $request);
+
+      $dialog = id(new AphrontDialogView())
+        ->setUser($user)
+        ->setTitle(pht('Entering High Security'))
+        ->setShortTitle(pht('Security Checkpoint'))
+        ->setWidth(AphrontDialogView::WIDTH_FORM)
+        ->addHiddenInput(AphrontRequest::TYPE_HISEC, true)
+        ->setErrors(
+          array(
+            pht(
+              'You are taking an action which requires you to enter '.
+              'high security.'),
+          ))
+        ->appendParagraph(
+          pht(
+            'High security mode helps protect your account from security '.
+            'threats, like session theft or someone messing with your stuff '.
+            'while you\'re grabbing a coffee. To enter high security mode, '.
+            'confirm your credentials.'))
+        ->appendChild($form->buildLayoutView())
+        ->appendParagraph(
+          pht(
+            'Your account will remain in high security mode for a short '.
+            'period of time. When you are finished taking sensitive '.
+            'actions, you should leave high security.'))
+        ->setSubmitURI($request->getPath())
+        ->addCancelButton($ex->getCancelURI())
+        ->addSubmitButton(pht('Enter High Security'));
+
+      foreach ($request->getPassthroughRequestParameters() as $key => $value) {
+        $dialog->addHiddenInput($key, $value);
+      }
+
+      $response = new AphrontDialogResponse();
+      $response->setDialog($dialog);
+      return $response;
+    }
+
     if ($ex instanceof PhabricatorPolicyException) {
 
       if (!$user->isLoggedIn()) {