(New flow) Log in and register via Magic links

[marfavi]

LoginFlow would be replaced by a email text field.
A magic link is always sent to that email.

• If that email belongs to an account, the link will log the user in.
• Otherwise, the link will trigger an account registration.

This will retire the use of passcodes.

When a user registers using a magic link, they will skip the current two screens of the registration flow, as the email is already known (and passcodes will retire). The user would just need to provide their name and programme (#124)