support Bootstrap A11y plugin

[andrewmallis]

https://github.com/paypal/bootstrap-accessibility-plugin
makes many elements more accessible.
We've contributed to this.

Why not include it defacto if we're going to be a BS theme?

[RobLoach]

Bootstap 4 has some accessibility built in.

[andrewmallis]

Does BS4 this make the plugin irrelevant @RobLoach ?