Draft State Quizzes: show multiple due dates

Closes CNVS-9880

This patch enables observers watching students across multiple sections
to see each section's due date for a quiz in the quizzes index page,
it also fixes the dates students get to see when they're in a section
other than the base one.

(BREAKING?) API CHANGES
----------- --- -------

  - when a student queries a quiz, the `due_at`, `lock_at`, and
    `unlock_at` dates they receive are that of the section they're in as
    oppossed to the quiz's global dates
  - when an observer queries a quiz, they receive the dates for the
    sections they're bound to in the `all_dates` field like teachers do

TEST PLAN
---- ----

Two ways to test this patch: a simple way that tests only the case
described by the ticket, or the comprehensive way.

> The simple way

  - turn DS on
  - create a course with multiple sections
  - create a quiz and assign a due date to each section
  - as an observer who's watching more than 1 student in different
    sections, go to the quizzes index:
    - verify that you see "Multiple Dates" for due dates (and
      availability if you set them)
    - hover over the link and verify that you see the proper dates in
      the tooltip

> The comprehensive way

Check this out: https://gist.github.com/amireh/375171767da8303e1b71

Change-Id: I934cb47f0229a43713dc6b4a6d280c047a2263b9
Reviewed-on: https://gerrit.instructure.com/30083
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>