add submission inclusion for students in polling

fixes CNVS-13826
This commit embeds a student's own submission into the poll sessions
endpoints.

Test plan
1. As a teacher, create a poll with choices and an opened session.
2. As a student, submit a submission to the poll session.
3. As the student, request the poll sessions GET show endpoint.
- The student's submission should be available in the "poll_submissions"
  field.

Other considerations are privacy: a student shouldn't be able to see
others' submissions (unless the poll session has public results).

Change-Id: I6050d5597fd44c0c0e9672b8e69a38022a3ea22d
Reviewed-on: https://gerrit.instructure.com/37078
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>