Implement 2-factor authentication

[sync-by-unito]

Created by: DanAtkinson

Placeholder issue to discuss implementing a mechanism for 2-factor authentication. This isn't a requirement (could be optional) for standard users, but becomes more important for users with elevated permissions.

┆Issue is synchronized with this Gitlab issue by Unito

[TheCakeIsNaOH]

I would generally prefer having TOTP and email available as options.
Both of these options are not locked into a single provider/app/hardware device, and so should continue to work indefinitely, instead of being tied to the continued existence of a specific company. For TOPT there are a bunch of apps available on mobile devices, and there are desktop applications available, and there are hardware devices that are compatible with it.