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/controllers/api/v1/events_controller.rb

@@ -5,7 +5,7 @@ module Api
       respond_to :json
 
       def index
-        events = Event.includes(:track, :room, :event_type, event_users: :user)
+        events = Event.includes(:track, :event_type, event_users: :user)
 
         if @conference
           events = events.where(program: @conference.program)