New associations and models: schedule versioning

For the schedule versioning in the admin interface (having multiple possible shcedules) we need to change some models and associations. It has also been create a rake:move_events_attributes task to migrate the data.

User Interface addapted to make use of the introduced associations and models

app/views/conference_registrations/_registration_info.html.haml

@@ -11,7 +11,7 @@
         .text-muted
           = registered_text(event)
           - if event.scheduled?
-            (Scheduled on: #{event.start_time.to_date})
+            (Scheduled on: #{event.scheduled_start_time.to_date})
       %br