Ember Quiz Stats - Show a loading spinner

This patch makes it so when you click the Statistics tab, and had not
already loaded its data, a dark overlay with a loading spinner will show
up to indicate that stuff is loading.

Closes CNVS-13737

TEST PLAN
---- ----

  - enable NQS
  - go to ember quizzes show
    + click the statistics tab
      - verify you see a dark overlay with a loading spinner
    + go back and forth between Overview and Statistics
      - verify you no longer see the overlay

Change-Id: I5d74e3093d222fe5ee9e5e037fb5fe64af464de3
Reviewed-on: https://gerrit.instructure.com/38346
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>