add has_overrides to assignments api

closes CNVS-17620

test plan:
 - assignments api (both index and show) should have 'has_overrides' flag
   that accurately reflects whether the assignment has overrides

Change-Id: Ifc74c955c3404552631a5ce435aee7bd98f5dcce
Reviewed-on: https://gerrit.instructure.com/46176
Reviewed-by: Matt Berns <mberns@instructure.com>
Reviewed-by: Benjamin Porter <bporter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>