Original email address is shown to admin users in subscriber list when
email_address_visibilty is set to "Admins only" by passing delivery_email
at required places. Email address are not shown to non-admin users when
visibility is set to "Admins only".
Tweaked by tabbott to fix a few bugs and dead code.
Fixes a part of #13541.
User IDs are more robust than email addresses as they don't change
with time, and also don't have complications with
different email_address_visibility settings.