[feature] Support multi-factor authentication

[ghost]

This could be useful especially in multi-tenant and commercial installations to help protect customer data. Ideally this should support authenticator apps. I have seen some Django packages such as this one that may be able to deliver a turnkey solution.

Documentation

[nemesifier]

There's high chances that this feature does not need any new code to be added to OpenWISP in order to work but this needs to be confirmed.