Add a stable ID to the user endpoint when asking for profile (BB-8580)

As BB API consumers, how can we implement a user migration when a user decides to change both his username and hit email address?

Could you add the stable ID of the user to the /user endpoint?

Without it, BB users are not satisfied using out platform with a BB account because, when such a change happens, they have to start from scratch (like new users). Github users benefit of a transparent migration feature because the Github API returns this stable ID in each profile.