Add Assignment created/updated_at to the API

closes CNVS-13682

Test Plan:
- Ensure there is a assignment in your test instance
- Request the assignment from the API
- The API response should include the created_at and updated_at elements
  in the returned JSON

Change-Id: Ib3067c9b204be95aa890d783de629162f4cc01f9
Reviewed-on: https://gerrit.instructure.com/36625
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>