Why:
Both Newsletters and Email Downloads need the same logic: To extract the
emails from all the users in the segment that have newsletter flag
active, removing all empty email values.
How:
1- UserSegments#user_segment_emails holds that repeated logic and is used
on both Newsletter & EmailDownload.
2- Rename Newsletter#list_of_recipients to list_of_recipient_emails as
it is more descriptive. There is no need to pass entire Users around,
only the emails are needed at Mailer#newsletter method.
3- Cleanup Newsletter#list_of_recipient_emails model spec scenario
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