add role="tabpanel" to .panel-collapse to improve accessibility

Refs #13555.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#collapse
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]
Merge pull request #14088 from twbs/panel-collapse-role-tabpanel

add role="tabpanel" to .panel-collapse to improve accessibility