Add role="tabpanel" to .tab-pane <div>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 #14086 from twbs/tab-pane-role-tabpanel

Add role="tabpanel" to .tab-pane <div>s to improve accessibility