add "id" to "all_dates" in quiz and assignment APIs

Test Plan:
  - Make a quiz and an assignment. Add some assignment overrides to
    each.
  - Query the quizzes show API for the quiz you made. You should see
    an "id" field for dates in "all_dates" that came from
    assignment overrides. For the "Everyone Else" date, it should
    not have an "id" field but should have a "base" field with the
    value of `true`.
  - Query the assignments API for the same information for the
    assignment you made. The "id" field should be present for
    dates that came from assignment overrides. For the
    "Everyone Else field" date, it should not have an "id"
    field, but should have a "base" field with the value of
    `true`.

closes CNVS-11445

Change-Id: I4136b86673cf772149b7735c0fe9c6730ff32415
Reviewed-on: https://gerrit.instructure.com/30787
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>