api: restore regular per_page defaults to assignment index action

closes CNVS-14424

test plan:
- in a course with >10 assignments, list them with the api
- you should get 10 back by default

Change-Id: If671da72e1f6f85bce57635460306123addcaeb2
Reviewed-on: https://gerrit.instructure.com/38232
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>