CQS: accessible summary stats table

Closes CNVS-15310

TEST PLAN
---- ----

  - go to statistics cqs page
  - turn on screen-reader
  - locate the table and verify that it reads in a sensible manner
  - verify you hear a short description of what the table is when you
    get to it
  - verify that no column header is abbreviated anymore, so "Std.
    Deviation" now reads "Standard Deviation" and "Avg. Time" now reads
    "Average Time"
  - verify the last column for the Average Time reads a
    human-understandable duration, like 10 seconds or 5 minutes and 30
    seconds instead of zero-zero-colon-one-zero (00:10)
  - sighted users should still see the duration in timestamp-form, e.g,
    00:10 and 05:30

Change-Id: Ia15877f654a4538e939758fcf89ad45f36e52df2
Reviewed-on: https://gerrit.instructure.com/40686
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>