Improved OAuth2 registration flow

OAuth2 flows initiated with the prompt=registration flag now function correctly when a user has previously completed an login or registration OAuth2 flow, and the project has session.cookie.persistent: true.

Additionally, when a user has an existing session but hasn't undergone an OAuth2 flow before, or the project configuration is set to session.cookie.persistent: false, then the user will be redirected to a new login flow, prompting them to re-authenticate.