Extract method to get a user segment name

We're going to add geozones as user segments, so it's handy to have the
method in the UserSegments class.

We're also changing the `user_segment_emails` parameter name for
consistency and simplicity.

spec/system/admin/admin_notifications_spec.rb

@@ -215,15 +215,15 @@ describe "Admin Notifications", :admin do
   end
 
   scenario "Select list of users to send notification" do
-    UserSegments.segments.each do |user_segment|
-      segment_recipient = I18n.t("admin.segment_recipient.#{user_segment}")
+    UserSegments.segments.each do |segment|
+      segment_recipient = UserSegments.segment_name(segment)
 
       visit new_admin_admin_notification_path
 
       fill_in_admin_notification_form(segment_recipient: segment_recipient)
       click_button "Create notification"
 
-      expect(page).to have_content(I18n.t("admin.segment_recipient.#{user_segment}"))
+      expect(page).to have_content segment_recipient
     end
   end
 end