LP#1305958 Change copy table header atts to scope attributes

Per http://webaim.org/techniques/tables/data, the current best practice for
marking up tables for accessibility is to use the "scope" attribute for
headers, rather than the error-prone and markup-heavy "headers" attribute on
table cells.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>
Signed-off-by: Ben Shum <bshum@biblio.org>