Introduce an Admin::EventScheduleController

- move SchedulesController#update action
- split it into an #create/#update and #destroy action and use these actions in the JS code

app/views/admin/schedule/show.html.haml

@@ -18,7 +18,7 @@
       Unscheduled events
     .unscheduled-events
       - @unscheduled_events.each do |e|
-        = render partial: 'event', locals: { event: e }
+        = render partial: 'event', locals: { event: e, event_schedule_id: nil }
   .col-md-10
     %ul.nav.nav-tabs
       - @dates.each do |date|