Add UserSegments#user_segment_emails helper method

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

lib/user_segments.rb

@@ -37,6 +37,10 @@ class UserSegments
     author_ids(current_budget_investments.winners.pluck(:author_id).uniq)
   end
 
+  def self.user_segment_emails(users_segment)
+    UserSegments.send(users_segment).newsletter.pluck(:email).compact
+  end
+
   private
 
   def self.current_budget_investments