add models for polling

fixes CNVS-12467
This commit adds the models for the new polling feature: Poll,
PollSubmission, and PollChoice.

Change-Id: If8adb91073d8173b14206f442fadbee4a9e3b4c9
Reviewed-on: https://gerrit.instructure.com/33566
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>