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/events/registrations.html.haml

@@ -11,7 +11,7 @@
         for
         = @event.title
 
-    - if @event.room && (@event_registrations.length > @event.room.size)
+    - if @event.scheduled_room && (@event_registrations.length > @event.scheduled_room.size)
       %b Attention:
       You have more registrations than the capacity of the room!