Previously, in cases when skip_reconfirmation! was called before
updating the primary email, the email (which is to be considered
confirmed) would not be added to the emails table, because
`User#confirm` was not called.

Now instead of overriding `User#confirm` we use an `after_save` callback
that checks if the primary email changed and if it is confirmed, and if
so adds it to the emails. This works also in case skip_reconfirmation is
called, and is also cleaner than overriding a Devise method.