Support multiple devices per user

[clokep]

I've seen sites that allow a user to configure multiple 2FA devices. This could be a reasonable feature to support!

[9mido]

#109

This PR would help with this because a user could manually add the secret key to their multiple device OTP applications like google authenticator/andOTP/aegis or multiple yubikey devices. The user would need to setup all of their devices prior to submitting the django-allauth-2fa setup page form with the verification code.

But what this PR will not do is show the user on your website how many devices they have currently setup or what the devices actually are.