paginate the course list api collection

test plan:
 - have a user enrolled in >10 courses
 - hit the /api/v1/courses endpoint as this user
 - verify that only the first 10 courses are returned
 - verify that you can use ?page=2 to get the next page
   of results

fixes CNVS-10763

Change-Id: I1eae1a46d5c7b049fa19af6a802ab4d9fa499d2b
Reviewed-on: https://gerrit.instructure.com/29542
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>