Fixes: #472

Need it to escape quirks mode in email clients.

Fixes: #347

Fixes: #348

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.

Add a "Send Nag" admin command for easier testing.

This avoids duplicate calls to Check.get_grace_start() in several places.

Apply the user's chosen sort order in emails also.