New user cannot change password

#1967936: Redirect to master a password reset request from a slave does a great job describing the problem. Also it proposes a solution (only register on the master, never on the slave). I no longer think that is the best approach.

Instead, I think the master should allow a user to change their password without knowing the current password, but only when bakery created the new account and the user has not yet changed the password. The attached patch attempts this behavior.