UI for quiz submission logs

Closes CNVS-17020

Adds a small app for browsing quiz submission events for a student's
submission. Re-architectures the previous canvas_quiz_statistics client
app to now host more than 1 app. Common code between the two apps is
factored out into /apps/common/{css,js,test}.

Also:

  - a feature flag for quiz log auditing
  - renamed previous QuizSubmissionEventsController to an Api one
  - an API endpoint + controller action for retrieving/viewing events
  - "canvas_quiz_statistics" merged with the new client app
    "canvas_quizzes"

TEST PLAN
---- ----

Make sure you build the assets by running `bundle exec rake
canvas:compile_assets` before doing any of this.

Now:

  - take a quiz as a student
  - as a teacher, view the student's attempt
  - click on the big "View Log" link
    + verify the page loads and you can browse around in it

We also need to test stats, because the commit affects it:

  - turn on the new stats feature flag
  - go to stats:
    + verify the page loads and looks OK

Try both pages another time with the ?optimized_js=1 parameter added to
the URL to make sure nothing went wrong when they were optimized.

Change-Id: I5a30cb0db05e80084d1ddd595b53f9aa3cf336eb
Reviewed-on: https://gerrit.instructure.com/44576
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>