bring API docs up to swagger spec

- conversations_controller
  - "properties", "audience", "audience_contexts", and
    "participants" were incorrectly using "type": "[string]"

- discussion_topics_controller
  - FileAttachment and DiscussionTopic ids were missing

- quizzes/quiz_submission_questions_controller
  - "answer" was missing a datatype

- sis_imports_api_controller
  - spec does not support arrays of arrays, so use array of
    strings for now

- submissions_api_controller
  - add Submission @model
  - reference the Submission model in return type of List-
    Submissions

- submissions_controller
  - remove Submission @model since it is not reference in this
    file (and must be referenced at least once for YARD to
    include it in its output)

Test Plan:
- make sure above changes don't break anything in API docs

Fixes SIS-648

Change-Id: I6c051c1e0ee58688b10f964272918c53b7e82c34
Reviewed-on: https://gerrit.instructure.com/44247
Reviewed-by: Ken Romney <kromney@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>