reuse assignment groups with matching name on import

test plan:
 - create a course (A)
 - put an assignment into A's default assignment group
 - create another course (B)
 - import course A into course B
 - confirm that B has only one assignment group named "Assignments"
   (no empty duplicate "Assignments" group)

fixes CNVS-17498

Change-Id: I223a0762d2f3e10635af788303c0510ae36cc0b7
Reviewed-on: https://gerrit.instructure.com/46147
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>