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