Quiz Submissions API - Self-Access

Passing a string id of "self" now returns the user's quiz submission.

Closes CNVS-10074

TEST PLAN
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Create a quiz and take it, and then using the API do the requests below:

[GET] /api/v1/courses/:course_id/quizzes/:quiz_id/submissions/self

  - verify that you can access your own quiz submission

[GET] /api/v1/courses/:course_id/quizzes/:quiz_id/submissions

  - verify that you get a list of submissions containing 1: your own

Change-Id: I8e98fd2ae081b7ff38f0bc7eaa6f3910241491a3
Reviewed-on: https://gerrit.instructure.com/28966
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Reviewed-by: Stanley Stuart <stanley@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>