Planning to use it for sensitive operations (add/remove security keys),
change email, change password, close account.

The decorator sends a six-digit confirmation code to user's email
and renders a form for entering it back. If the user enters the
correct code, the decorators sets a sudo=active marker in
user's session, valid for 30 minutes.

Resolves #408 by fixing the test_signup logic bug introduced in 8c13457.

The team size limit is applied to the number of distinct users across all projects. Fixes #332.