only copy external feeds when selected

test plan:
 - create a course with an external feed (announcements page)
   and an assignment
 - copy the course. the copy should contain both.
 - copy the course, but select only the assignment. the copy
   should not contain the external feed.

fixes CNVS-14773

Change-Id: I321522c143e84a50d6a570967cb98505a8011800
Reviewed-on: https://gerrit.instructure.com/39393
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>