Do not use JS to initialize the schedule

- Only use JS for the drag&drop functionality and to the set/alter the time and rooms of the EventSchedule object.

- Introduce has_many :events, through: :event_schedules association to get unscheduled event easily

app/serializers/event_serializer.rb

@@ -34,7 +34,6 @@ class EventSerializer < ActiveModel::Serializer
     end
   end
 
-  # FIXME: duplicated logic from Event#as_json
   def length
     object.event_type.try(:length) || EventType::LENGTH_STEP
   end