CQS - Loading states

While stats are being loaded:

  - Adds a spinner to the top of the page
  - Dims the summary table metrics and replaces them with "N/A"
  - Reads a "please wait while we're loading" sentence in the question
    breakdown section
  - Disables any buttons that are visible

Note: the summary percentile chart could use some work but it'll have to
be done in a different patch.

Closes CNVS-16597

TEST PLAN
---- ----

  - go to new quiz stats
  - try to get the page to load for a bit so you can see the effects,
    otherwise try reloading to test each item or get in touch and maybe
    we can figure something out
    + verify the items above are met while the stats are being loaded
    + verify everything goes back to normal when loading is complete

Change-Id: Ibd9049d3a81d85d376b79102c1567e470f06de9c
Reviewed-on: https://gerrit.instructure.com/43997
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>