Quiz Stats - File Upload & Formula

Add support for generating stats for File Upload and Formula questions.
Similar to Essay metrics but adjusted to calculate properly.

Closes CNVS-13169

TEST PLAN
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  - create a quiz with those question types
  - take the quiz:
    - answer both question types by at least one student, but leave it
      unanswered by another so we can test the "responses" metric
  - test the stats:
    GET /api/v1/courses/:course_id/quizzes/:quiz_id/statistics
    - the "responses" metric should count the number of students who
      provided an answer
    - the "graded" metric should read the number of students whose
      answers you've graded so far
    - the "full_credit" one should read 0 until you grade it and give
      them a score higher than, or equal to, the maximum points possible
    - "point_distribution" is similar to that of Essays; it is an array
      of objects that track every score you gave the students and the
      number of students who received those scores
  - grade the scores and re-test to verify the metrics update correctly
  - verify that the API documentation is updated to include those
    question types

PS: the "responses" field will read "the number of students who uploaded
a file" for File Upload, and "the number of students who wrote any
answer" for Formula.

Change-Id: I890eafe018e000eef88de782bd7e86b5259df5d5
Reviewed-on: https://gerrit.instructure.com/35112
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>