FIX: Suggest current username for staged users (PR #13706)

If user had a staged account and logged in using a third party service a different username was suggested. This change will try to use the username given by the authentication provider first, then the current staged username and last suggest a new one.

GitHub

I don’t think we need this branch… it’s already covered by the logic in User#username_available?

I checked it, I think that we need that fallback in case the username is not provided by the authenticator