Refactored model code for better readability

conditional_user_mailer concern for delivering email according to preferences. DMPRoadmap/roadmap#811 ...

unit test for the concern. DMPRoadmap/roadmap#811

deliver_if recipients keyword accepts User or Array of Users. DMPRoadmap/roadmap#811

conditional email for users.new_comment, owners_and_coowners.visibility_changed. DMPRoadmap/roadmap#811

conditional email for users.added_as_coowner. DMPRoadmap/roadmap#811

conditional email for admins.feedback_requested. DMPRoadmap/roadmap#811