don't copy all the grading standards in every course copy

test plan:
 - set a grading standard for a course
 - link a new grading standard to an assignment in the course
 - also have an assignment without a grading standard
 - and a grading standard that's unused
 - copy the course (without selecting content)
   - the copy should have all the grading standards from the original
 - copy the course, selecting only the assignment with no grading standard
   - the copy should have no grading standards
 - copy the course, selecting only the assignment with the grading standard
   - only the assignment's grading standard should be in the new course
 - copy the course, selecting only course settings
   - only the course's grading standard should be in the new course

fixes CNVS-5499

Change-Id: I3899769e960b17869245ebf9a5a8c4811755c362
Reviewed-on: https://gerrit.instructure.com/30775
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>