exclude grading schemes/assignment groups in selective assignment export

test plan:
 0. have a course containing an assignment with a custom grading scheme
    and named assignment group. also a graded discussion topic in a
    different assignment group with a different grading scheme, and
    a published quiz in a third assignment group.
 1. copy the course. the destination course should copy the assignment
    groups and grading schemes and associate them with the copied objects
    correctly.
 2. repeat step 1, but do a selective course copy, and select the
    assignment, quiz, and topic specifically. you should get the
    same result as in step 1.
 3. use the content exports API to selectively export the three objects
    e.g.,
      select[assignments][]=X&select[quizzes][]=Y&select[discussion_topics][]=Z
    import this package into a new course. the three objects should be
    there, but should not have custom assignment groups or grading
    schemes.

fixes CNVS-14820

Change-Id: I871771284b4a3bbd3695a99a6f2af613d76a8ebf
Reviewed-on: https://gerrit.instructure.com/39914
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>