R1:28ee6b8080fc

Consistently require MFA on the actual user creation flow

Summary:
See <https://hackerone.com/reports/351361>. We currently require MFA on the screen leading into the user create flow, but not the actual create flow.

That is, `/people/create/` (which is just a "choose a type of account" page) requires MFA, but `/people/new/<type>/` does not, even though this is the actual creation page.

Requiring MFA to create users isn't especially critical: creating users isn't really a dangerous action. The major…