fix assignment group re-rendering on initial load

don't render on add until the collection is loaded

test plan:
* create a course with over 10 assignment groups
* it should work (and also load much better)

closes #CNVS-24118

Change-Id: I7c12e43042a2e0b0587e2a86fd4a9813c6cade28
Reviewed-on: https://gerrit.instructure.com/66096


Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Michael Hargiss <mhargiss@instructure.com>
Product-Review: Jason Sparks <jsparks@instructure.com>

app/coffeescripts/views/assignments/AssignmentGroupListView.coffee

@@ -62,10 +62,14 @@ define [
       super
       @itemViewOptions = course: @course
       @collection.on 'render', @render
-      @collection.on 'add', @render
+      @collection.on 'add', @renderIfLoaded
       @collection.on 'remove', @render
       @collection.on 'change:groupWeights', @render
 
+    renderIfLoaded: =>
+      if @collection.loadedAll
+        @render()
+
     renderOnReset: =>
       @firstResetLanded = true
       super