add child attachments association

Change-Id: I5c4f55aa5f4af8d05a13937735c0fc61603a1cbe
Reviewed-on: https://gerrit.instructure.com/52608
Tested-by: Jenkins
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/attachment.rb

@@ -63,6 +63,7 @@ class Attachment < ActiveRecord::Base
   has_one :sis_batch
   has_one :thumbnail, :foreign_key => "parent_id", :conditions => {:thumbnail => "thumb"}
   has_many :thumbnails, :foreign_key => "parent_id"
+  has_many :children, foreign_key: :root_attachment_id, class_name: 'Attachment'
   has_one :crocodoc_document
   has_one :canvadoc
   belongs_to :usage_rights