[FEATURE] Generate anonymous account based on a public link / QR code - Self-signup

[bardram]

Now we support that a user can scan a QR code or URL link for a specific anonymous account. But each link/QR is specific to an account.

We need a feature where one QR code allows both creating the account and logging in. In this way, we can distribute a single QR code for a study and have users self-sign up.