MFITB question validation fixes
MFITB question validation fixes

MFITB question types were beginning to be validated, but they were
broken in certain edge cases.  This commit approaches it in a less edge
case-y way by examining each question individually.

Closes CNVS-17619

Test Plan:
  - Make a quiz question which contains a MFITB question type
    - Try to save it in the following states:
      - Two blanks with [no_answer, one_answer] for each blank
      - Three blanks with [no_answer, one_answer, two_answers] for each
        blank
      - Go for broke with 10 blanks with [no_answer...four_answers] for
        each blank
  - Confirm there are no problems with errors on short answer questions,
    in that submitting blank answers are prevented.

Change-Id: I8c8e916b2c0fb42ec7aa50b35be77a62b7f67d6a
Reviewed-on: https://gerrit.instructure.com/46542
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Josh Simpson <bfb4987b134a00f73d881fff5113b1d5fa1876bc@instructure.com>
QA-Review: Trevor deHaan <85df72112e8a9484fec7b3db2e31789c2c2d4e07@instructure.com>
Product-Review: Matt Fairbourn <5f353e1c97bb9cc046b9db20beca0b9392f6f1b2@instructure.com>

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