Use regular javascript attributes for parent reference rather than setting

Backbone.Model attributes because Model attributes get serialized into
JSON which is wasteful.

lms/static/coffee/src/discussion/views/response_comment_view.coffee

@@ -10,10 +10,10 @@ if Backbone?
     render: ->
       @template = _.template($("#response-comment-template").html())
       params = @model.toJSON()
-      params['deep'] = @model.has('parent')
-      if @model.has('parent')
-        params['parent_id'] = @model.get('parent').id
-        params['parent_username'] = @model.get('parent').get('username')
+      params['deep'] = @model.hasOwnProperty('parent')
+      if @model.hasOwnProperty('parent')
+        params['parent_id'] = @model.parent.id
+        params['parent_username'] = @model.parent.get('username')
       @$el.html(@template(params))
       @initLocal()
       @delegateEvents()

lms/static/coffee/src/discussion/views/thread_response_view.coffee

@@ -34,7 +34,7 @@ if Backbone?
         comments.add(comment)
         children = new Comments(comment.get('children'))
         children.each (child) ->
-          child.set('parent', comment)
+          child.parent = comment
           collectComments(child)
       @model.get('comments').each collectComments
       comments.each (comment) => @renderComment(comment, false, null)