add role="presentation" to tab <li>s to improve accessibility

Refs #13554.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]
Merge pull request #14085 from twbs/tab-lis-role-preso

add role="presentation" to tab <li>s to improve accessibility