assignments API: include rubric ID and title

fixes CNVS-10935

test plan:
  * create an assignment with a rubric
  * open http://localhost:3000/api/v1/courses/:course_id/assignments
  * verify that the assignment's rubric_settings includes the ID and title

Change-Id: I173ee4fac460633f7a04a7b83a392c9de74f3c7a
Reviewed-on: https://gerrit.instructure.com/29623
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>