fix 'let students take this quiz now' so that it correctly unlocks the quiz

when unlocking a quiz (with no time limit or for a specified period), the
quiz will now unlock correctly, and the students will be able to take it.

fixes CNVS-11727

test plan:
  - Create a quiz. Set a due date, but no "from" or "until" date.
  - Save and publish the quiz.
  - Lock the quiz; note that the date/time at which you lock it now appears in the "until" field.
  - Unlock the quiz with "No time limit". Note that the current date/time appears in the
      "Available from" cell, and the "Until" date has been cleared out.
  - Verify that you are able to take the quiz as a student, and note that there is no
      specified end date for the quiz.
  - Lock the quiz again, then unlock the quiz with a time limit (choose any date). Note that
      the current date appears in the "Available from" cell, and the specified end date
      appears in the "Until" cell.
  - Verify that you are able to take the quiz as a student, and note that there is a specified
      end date for the quiz.

Change-Id: I94ed3e26b7c91e84728c27e58c0e130444ef95a2
Reviewed-on: https://gerrit.instructure.com/42016
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>