Add enabled to event_schedules

To show scheduled events that have been cancelled/withdrawn as 'cancelled', and consequently also show when an event is their replacement.

admin/schedules#show does not show events that were scheduled and then cancelled/withdrawn, however a record in event_schedules exists for those events, but event_schedule.enabled = false

db/migrate/20170212145523_add_enabled_to_event_schedules.rb

@@ -0,0 +1,5 @@
+class AddEnabledToEventSchedules < ActiveRecord::Migration
+  def change
+    add_column :event_schedules, :enabled, :boolean, default: true
+  end
+end