QR Login: Detect if 2FA is turned on and warn user

QR Login isn't available for accounts with 2FA. It would be nice if we could detect if 2FA is enabled and warn the user.

• Slack ref: p1664879201895099/1664874812.012369-slack-C0180B5PRJ4
• iOS issue: https://github.com/wordpress-mobile/WordPress-iOS/issues/19397

Options
(1) Handle this on the clients before/after scanning. The QR code login flow is not required to start within the app; the device camera or a scanning app can start the flow.

• two_step_enabled is part of AccountSettings and does not exist in the AccountModel. FluxC changes are required to add this information to the AccountModel.
• The check for 2FA disabled can be made after "Scan Login Code" is tapped on the Me menu (before it goes into scanning mode). Show an error toast.
• If the QR code is scanned from the camera, not Me -> Scan Login Code, the user gets dumped into the scan flow and will see an error message. We'll need to adjust the error message about 2FA

OR
(2) Enhance the endpoint and return a new error code to indicate that 2FA is not enabled. Use this code to show an explicit "2FA" error message after scanning (the error view already exists)

OR
(3) An authentication error view is currently show in the QR login flow when auth fails. Update the web site to include a message that 2FA enabled accounts are not eligible to use the qr code login process. This messaging can be added to the "Login via the mobile app" view. This option requires no client changes.

Start by reading the QR login flow, the Me menu's "Scan Login Code" entry, AccountSettings, AccountModel, FluxC, and the existing authentication error view. The issue presents three possible approaches, so confirm the intended client, endpoint, or website scope before changing anything. Done should include a clear warning for accounts with 2FA enabled, including QR scans started outside the Me menu.