Adds scope to th tags on course roster

Adds screenreader-only text to the empty table headings

closes CNVS-15088, CNVS-15161

test plan:
  - Go to the people page (/courses/##/users)
  - Viewing the source should show scope attributes on the th's
  - Presumably a screenreader or other assistive technology will
    work better with this.

Change-Id: I802c7650a5bf409bd431b21da0d970252941cdae
Reviewed-on: https://gerrit.instructure.com/40192
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>