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/proposal/_proposal_form.html.haml

@@ -49,7 +49,7 @@
 
     = f.inputs 'Enable pre-registration' do
       = f.input :require_registration, label: 'Require participants to register to your event'
-      - message = @event.room ? "Value must be between 1 and #{@event.room.size}" : 'Check room capacity after scheduling.'
+      - message = @event.scheduled_room ? "Value must be between 1 and #{@event.scheduled_room.size}" : 'Check room capacity after scheduling.'
       = f.input :max_attendees, hint: 'The maximum number of participants. ' + message
 
     - if current_user.has_any_role? :admin, { name: :organizer, resource: @conference }, { name: :cfp, resource: @conference }