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/admin/tracks/show.html.haml

@@ -27,5 +27,4 @@
             %td
               = event.state
             %td
-              = event.start_time
-
+              = event.scheduled_start_time