add role="button" to [data-slide="prev"],[data-slide="next"] to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
Merge pull request #13559 from twbs/carousel-role

add role="button" to [data-slide="prev"],[data-slide="next"] to improve accessibility