add user_id to poll serializer

fixes CNVS-13360
This commit adds the user_id to the poll json response, only for
teachers.

Test plan
- As a teacher, create a poll
- Request that poll via the poll#show endpoint
- It should have a 'user_id' field with the id of the creator
- As a student, request the same poll via the poll#show endpoint (the
  student won't be able to view it without a published poll session)
- The student should not see the 'user_id' field.

Change-Id: Ib0b09d779f3486edaa38a870d20211268be819d3
Reviewed-on: https://gerrit.instructure.com/35693
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>