The list of emails for sending newsletters will always be ordered by
user creation date, so it will be easier to debug and to know for
which users the email has been sent.
Why:
UserSegments are not only used for Newsletters or Email downloads, but
also for internal Global Notifications. We don't want to have that scope
hardcoded inside UserSegments as users that have opted-out from the
newsletter should still be recipients of global notifications.
How:
Removing the scope from the UserSegments `all_users` method that acts as
base for all the other segments. Including that `newsletter` scope only
on the places that is relevant:
* When listing recipients for a newsletter
* When downloading a listing emails that can be newsletter recipients
Also updated relevant tests
