api: paginate the course assignments endpoint

closes CNVS-13475

test plan:
- create 10 assignments in a course
- call the 'list course assignments' endpoint with per_page=2 as a query
  parameter
- it should only return 2 assignments and have pagination links to go to the
  next page
- make sure those links work
- also try passing other allowed parameters like 'include[]', 'search_term' and
  'override_assignment_dates', and make sure they are maintained in the
  pagination links. in other words, make sure that if you are including
  submissions in the first call with the include[] parameters, subsequent calls
  to later pages also return the submission

Change-Id: Ib637eb95186a2d3c586aaca64ef3efe2d937b142
Reviewed-on: https://gerrit.instructure.com/37640
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>