Quizzes edit form: "Only Once" checkbox fix

A few changes to how the "Let Students See Their Quiz Responses" form
section works:

  - if the "Let Students See Their CAs" checkbox is on:
    + we show the date-pickers like usual
    + if the "Only Once" checkbox is turned on:
      - we still show them, but they're disabled (teacher can't use,
        dates don't go through to the backend)
  - if it's off:
    + we totally hide the date-pickers

The primary fix was that the "Only Once" checkbox is no longer tied to
the correct-answer visibility fields, but instead to the
result-visibility fields (which is consistent with how the form looks
like). The back-end will also ignore the showAt/hideAt dates if that
option is turned on.

Closes CNVS-14641

TEST PLAN
---- ----

  - go to edit quiz
  - verify the UI behavior is as described in the top of this message
  - uncheck the "Let Students See Their Correct Answers" option
  - check the "Only Once After Each Attempt" option
  - save
    + verify your options were saved properly

Change-Id: I4587bcb748e238bf351e90e6707e6244aa3289b7
Reviewed-on: https://gerrit.instructure.com/39190
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>