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
Why:
Newsletters without a valid user segment can't be sent since there is no
recipient user email list.
How:
* Hiding the send button at the form
* Preventing an invalid newsletter from being delivered at the controller
