add has_submitted to poll_session

fixes CNVS-13312
This commit adds a 'has_submitted' key to poll_session json.
The 'has_submitted' key lets the user requesting the poll_session know
whether or not they've submitted a submission to the requested
poll_session.

Test plan
- Create a poll and poll session
- Make an API call to poll_session#show as a student
  - 'has_submitted' should be false
- Make a submission to the poll session as a student
- Make an API call to poll_session#show as the same student
  - 'has_submitted' should be true

Change-Id: I189b1fca4c467019b51123e929c4f45e5cf2d227
Reviewed-on: https://gerrit.instructure.com/35571
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>