account statistics accessibility

fixes: CNVS-15344

Added a scope to the row headers in the table.

Test Plan:

- Using a screen reader, visit the first table on the page.
- Use the screen reader navigation keys to navigate through the table,
  and verify that as you move up and down in the second and third
  column, the screen reader reads the row heading from the left column.

Change-Id: I3bbe277c1bc19145df6b8922ccb6d67f89f9389a
Reviewed-on: https://gerrit.instructure.com/40845
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>