fixes course copy on concluded courses

fixes CNVS-13516

test plan
 - conclude a course or have course in an ended term
 - course copy from that course should work
 - here is api call to test
curl 'http://localhost:3000/api/v1/courses/2/content_migrations' \
-F 'migration_type=course_copy_importer' \
-F 'settings[source_course_id]=1' \
-H 'Authorization: Bearer <token>'

Change-Id: I4c09311eb9eb19ef58f5c0cba221bf505511fb41
Reviewed-on: https://gerrit.instructure.com/36329
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>