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
Why:
Newsletter attribute `segment_recipient` is an integer to be used as
enum. There's no advantage to store a number instead of an string if the
ammount of elements in the table is not going to be huge, or we can take
advantage of using an enum.
Also maintaining both Newsletters enum paired with UserSegments::SEGMENTS
would be a maintenance burden.
How:
* Migration to change segment_recipient column from integer to string
* Removing enumeration from Newsletter model class
* Using UserSegments::SEGMENTS instead of Newsletter.segment_recipients
or integer values
