[keycloak-dev] User ids and usernames

A user should have an id, username and email (what we have now). The id should be generated by the server and should never change for a user. The sub field in the token should use this id, not the username. Applications that wants to store information associated with a specific user should also use this id, not the username or email, as the id will never change. That means it should be possible for a user to change his/her username. Obviously a username has to be unique within a realm. We should then allow a user to login with either their username or their password. When a user is able to login with their username we can also remove the forgot username option on the login form, and only have a forgot password option. This would also help integration with social login as now we don't have to try to create a sensible username for a user on social login. Instead we create a generated id, and don't even set a username. A user can then set the username they want through the account management (or on the update profile action page if that option is enabled). If there's no objections to this, I'd like to add these changes to alpha2.