Add role="tablist" & aria-multiselectable="true" to accordion .panel-group 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 #14089 from twbs/accordion-panel-group-aria-multiselectable

Add role="tablist" & aria-multiselectable="true" to accordion .panel-group