embed poll submissions into poll session

fixes CNVS-13275, CNVS-13297
This commit embeds all submissions when a particular poll session is
requested by authorized users (the creator/teacher of the poll and
relevant poll session.).  It also deletes all submissions and sessions
that belong to a poll when that poll is deleted.

Test plan
Submission embedding
- Create a poll and related poll session
- Create multiple submissions for the poll as students
- As a teacher, you should be able to see the related submissions
  when requesting the poll session via the API

Poll session/submission deletion
- Create a poll and two poll sessions, with two submissions per session
- Delete the poll via the api, it should be deleted successfully

Change-Id: I335f92eb97be095a2ddf6bc730954ed8a14220ef
Reviewed-on: https://gerrit.instructure.com/35456
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>