Adds quiz taking session start event tracker

This captures the relevant user-agent string and a timestamp for the
initialization of a quiz taking event.

Refs CNVS-17138

Test-Plan:
  - Take a quiz with the '?debug_js=1' URL param
  - Confirm the user-agent string message is shown
  - Access the quiz from another tab and confirm message is present.

Change-Id: I46799b3405a955499fdd03138c5565b513d7920f
Reviewed-on: https://gerrit.instructure.com/45086
Reviewed-by: Ahmad Amireh <ahmad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>