Make many actions require high security

Summary:
Ref T4398. Protects these actions behind a security barrier:

  - Link external account.
  - Retrieve Conduit token.
  - Reveal Passphrase credential.
  - Create user.
  - Admin/de-admin user.
  - Rename user.
  - Show conduit certificate.
  - Make primary email.
  - Change password.
  - Change VCS password.
  - Add SSH key.
  - Generate SSH key.

Test Plan: Tried to take each action and was prompted for two-factor.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8921

src/applications/auth/controller/PhabricatorAuthLinkController.php

@@ -83,6 +83,11 @@ final class PhabricatorAuthLinkController
 
     switch ($this->action) {
       case 'link':
+        id(new PhabricatorAuthSessionEngine())->requireHighSecuritySession(
+          $viewer,
+          $request,
+          $panel_uri);
+
         $form = $provider->buildLinkForm($this);
         break;
       case 'refresh':