Feature: support multiple devices

[sergei-maertens]

Users do switch devices and may need to set up/register their new device and remove the old one. Currently it doesn't seem possible to add an additional device

• Allow adding additional devices (e.g. hardware token + TOTP)
• Allow selecting the device during login
• Allow removing devices (unless it's the last device)

Caution - implementing this will break the django-hijack "restore default device" behaviour! It may grab the wrong device to set in the session.