Fix Changing Course Todo to Personal Todo on Calendar

fixes COMMS-503

Test Plan
* Create a todo either in calendar or on planner screen
* Make thta todo tied to a course
* find the todo on calendar screen and click on it to go to edit
event modal
* Change the calendar to the personal calendar
* Watch how it actually removes the course

Change-Id: Id95f133598d4ec6e7a82ad15b42df91709d445f9
Reviewed-on: https://gerrit.instructure.com/132791
Reviewed-by: Venk Natarajan <vnatarajan@instructure.com>
QA-Review: KC Naegle <knaegle@instructure.com>
Tested-by: Jenkins
Product-Review: Matt Goodwin <mattg@instructure.com>

app/coffeescripts/calendar/EditPlannerNoteDetails.coffee

@@ -150,6 +150,8 @@ define [
         params.course_id = data.context_code.replace('course_', '')
       else
         # is in the user's calendar
+        if !@event.isNewEvent()
+          params.course_id = ""
         params.user_id = data.context_code.replace('user_', '')
 
       params