Refactor segment constant into a class method

We're going to make it dynamic using the geozones. Besides, class
methods can be overwritten using custom models, while constants can't be
overwritten without getting a warning [1].

Makes the definition of segments with geozones a little cleaner. I
think it’s worth it, compared to the slight memory gain of using a
constant [2].

[1] warning: already initialized constant UserSegments::SEGMENTS

[2] https://stackoverflow.com/questions/15903835/class-method-vs-constant-in-ruby-rails#answer-15903970

spec/system/admin/admin_notifications_spec.rb

@@ -215,7 +215,7 @@ describe "Admin Notifications", :admin do
   end
 
   scenario "Select list of users to send notification" do
-    UserSegments::SEGMENTS.each do |user_segment|
+    UserSegments.segments.each do |user_segment|
       segment_recipient = I18n.t("admin.segment_recipient.#{user_segment}")
 
       visit new_admin_admin_notification_path