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