Fixes #282

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.

When sending email using Django's default email
backend (SMTP), and if there is a network issue, the backend
can throw SMTPServerDisconnected.

This commit adds a retry logic which retries sending the
email two times when SMTPServerDisconnected is thrown.