Support two factor authentication

Multi factor auth (MFA) or two-factor auth (2FA) would be a nice addition to `hackage-server`. Both for its account management page and `cabal upload` functionality.

There seems to be three related libraries on Hackage:
- https://hackage.haskell.org/package/OTP
- https://hackage.haskell.org/package/crypto-totp
- https://hackage.haskell.org/package/gamgee (no go due to polysemy I guess)

As a minimal implementation, account management page would allow user to add a TOTP token via a QR code.

QR code related packages:
- https://hackage.haskell.org/package/qrcode-core
- https://hackage.haskell.org/package/qrcode-juicypixels

Suggestions welcome! I'm willing to work on this myself, would also appreciate co-authors since this requires a PRs for both hackage-server and cabal (and possibly for one of the OTP libraries).