CC-5641: Can Create Zero or Negative Length Shows

* Fixed two underlying causes of zero or negative length shows
  that could be created over DST time changes:
    - Fixed incorrect end time arithmetic which did not preserve the
      show duration.
    - Fixed a desynchronization issue due to FullCalendar handling
      events over DST differently from Airtime.

airtime_mvc/public/js/airtime/schedule/full-calendar-functions.js

@@ -325,6 +325,12 @@ function eventDrop(event, dayDelta, minuteDelta, allDay, revertFunc, jsEvent, ui
                 alert(json.error);
                 revertFunc();
             }
+            
+            //Workaround for cases where FullCalendar handles events over DST
+            //time changes in a different way than Airtime does. 
+            //(Airtime preserves show duration, FullCalendar doesn't.)
+            scheduleRefetchEvents(json);
+
         });
 }