add opened/closed endpoints for poll sessions

fixes CNVS-13262
This commit changes the external semantics for API users.  Instead of
'publish' and 'close', the two states of poll sessions are now 'open'
and 'close'.  This is reflected in the endpoints.

This commit also adds two endpoints for students: one to see all available
poll sessions (based on their enrollments) that are published, and
another to see all available poll sessions (based on their enrollments)
that are closed.

Test plan
- Create a poll, and multiple poll sessions for the poll that correspond
  to different courses.
- Publish the sessions
- As a student enrolled in one of the courses, the /published endpoint
  should return the poll session corresponding to the course they're
  enrolled in.
- The student should not see the sessions that correspond to the courses
  they're not enrolled in.
  - The student should not see any of the sessions via the /closed
    endpoint
- Close all sessions.
- The /published endpoint should not return any of the sessions.
- The /closed endpoint should return all sessions that correspond to
  course(s) the student is enrolled in.

Change-Id: Ib2743c123a2323c59d923264498f65cb60163405
Reviewed-on: https://gerrit.instructure.com/35503
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>