expose poll_choice#is_correct to students when available

fixes CNVS-13474
This commit exposes the is_correct attribute on poll_choices when a
student has submitted their answer to the latest poll_session, and that
poll_session has been closed.

Test plan
- As a teacher, create a poll, relevant poll choices, and a poll session
- Publish the poll session
- As a student, at this point is_correct should not be viewable on poll
  choices
- As a student, submit a poll submission
- As a student, at this point is_correct should not be viewable on poll
  choices
- As a teacher, close the poll session
- As a student, at this point is_correct should be viewable on poll
  choices

Change-Id: I20a8bb534affe5e1a23c1dddbb727043535dd23d
Reviewed-on: https://gerrit.instructure.com/36031
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>