Allow assignment with one valid override to save
Allow assignment with one valid override to save

Fixes: SIS-2862

Notes: Due to how the due date validations were implemented for
       assignments, an assignment created with an override that
       has a due date in a Course with multiple sections would not
       allow the assignment to save causing a 'Due Date Required'
       error to occur from the model.

Test plan:
- Enable SIS integration settings and check the require due date
  setting
- Navigate to a course that has at least 2 sections
- Create an assignment with one override and give the override a
  due date
- Confirm the assignment saves
- Confirm the other due date scenarios work as expected

Change-Id: I91cf729869b1926c9fab36407b9317dc9c35d2dc
Reviewed-on: https://gerrit.instructure.com/113798
Tested-by: Jenkins
Reviewed-by: Brad Humphrey <9233b6f5388d6867a2a7be14d8b4ba53c86cfde2@instructure.com>
QA-Review: Mark McDermott <20dd8ba615f2257ea7254f93d6f30ae11aa4f904@instructure.com>
Product-Review: Brad Humphrey <9233b6f5388d6867a2a7be14d8b4ba53c86cfde2@instructure.com>

djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms