export CC 1.3 assignment extension

test plan:
 - have an assignment in a course that has points possible
   and submission types
 - use the content exports API to export the course,
   supplying the currently undocumented parameter
   version=1.3
  a) examine the exported XML and verify that it follows
     the IMS standards
     http://www.imsglobal.org/cc/ccv1p3/AssignmentContentType.html
     http://www.imsglobal.org/cc/ccv1p3/CCv1p3Variantguidelines.pdf
  b) ensure the exported course re-imports correctly

fixes CNVS-13503

Change-Id: Iad8a54e94696962d362cfdb48fe03e06a90937ff
Reviewed-on: https://gerrit.instructure.com/36283
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>