Quiz Submission Questions - Bulk-answering

This patch makes the QSQ API accept answers to more than one question in
the same API call.

Closes CNVS-13667

TEST PLAN
---- ----

  - create a quiz with two or more questions
  - generate a submission, don't turn it in just yet
  - hit the API with a request similar to the example shown below:
    [POST] /api/v1/quiz_submissions/:id/questions
    - verify that the answers get through by checking the response of
      the API (should include the answers) and by turning in the quiz to
      make sure
  - verify the API docs r updated
  - other things you may want to test but are already covered by specs:
    - try hitting the endpoint against another student's submission,
      shouldn't work
    - try hitting the endpoint anonymously, shouldn't work either

Example request:

{
  "validation_token": "foobar",
  "attempt": 1,
  "quiz_questions": [{
    "id": "QUESTION_ID",
    "answer": "ANSWER_ID",
  }, {
    "id": "2",
    "answer": 42
  }]
}

The validation token is what you get when you generate the submission
(or just hit the API to retrieve the submission and it will include it
in the output.)

Change-Id: Iecec247efc4389883e49265005ffe77eae421a5c
Reviewed-on: https://gerrit.instructure.com/36559
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>