transition the polling api to the jsonapi standard

fixes CNVS-13271
this commit changes the formatting of the JSON returned by the polling
endpoints to conform to the adopted jsonapi standard. It also adds a
'created_at' field to the poll and poll_session endpoints

Test plan
- All polling endpoints should adhere to the jsonapi standard (nested
  under a collection, e.g. all returned JSON items will be contained under
  a pluralized root, polls will be under a 'polls' key, poll choices
  will be under a 'poll_choices' key, even for a single resource)

- There are no 'meta' or 'links' attributes to worry about yet

- The poll and poll_session endpoints should return a 'created_at'
  field.  This is viewable by both students and teachers

Change-Id: Ie0fd49a2c699fba5d8257ee1e8dbb062c81662cb
Reviewed-on: https://gerrit.instructure.com/35388
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ahmad Amireh <ahmad@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>