Consider adding accessibility plugin to the Bootstrap 4 bundle

[cpsievert]

Mainly so that sensible ARIA labels are added to navs and modals, but a better option might be to add ARIA labels to markup that shiny & rmarkdown generate to render the DOM walking here unnecessary

https://github.com/paypal/bootstrap-accessibility-plugin/blob/fbbf870eafc1ee5d4547fabbeea71778ddbe2166/src/js/tab.js#L4-L6