only update rubric association peer review links for assignments

fixes CNVS-10152

test plan:
- basic regression test with rubrics and peer-reviews
  * create rubric directly on the assignment vs create rubric on the course
    and then link to the assignment
  * with the above configuration, try changing the assignment rubric after
    peer reviews have already been assigned, but not completed.
  * have multiple assignments set up as above, and make sure that step 2 only
    affects the assignment that is linked to the new rubric (and not other
    assignments with outstanding peer reviews)

Change-Id: I553361f48c4d46ccff8609b5729b1e2737c5bb37
Reviewed-on: https://gerrit.instructure.com/27868
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
added validity check to assignment_group_id in assignments api

fixes CNVS-2123

Test Plan:
  - Add assignments via the API
    - if assignment[assignment_group_id] is a string, it should fail
    - if assignment[assignment_group_id] is blank, it should succeed
    - if assignment[assignment_group_id] is a number, it should succeed
    - if assignment[assignment_group_id] is not given, it should succeed

Change-Id: I810d050e638352d8138299954e9c1195513c1d96
Reviewed-on: https://gerrit.instructure.com/27745
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>