api: paginate the course assignments endpoint
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 <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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