Several updates to match stable Bootstrap 4 release.

bootstrap_barrio.theme

config/install/bootstrap_barrio.settings.yml

css/components/alerts.css

templates/block/block--breadcrumb.html.twig

templates/block/block.html.twig

templates/layout/page.html.twig

templates/layout/region.html.twig

templates/misc/status-messages.html.twig

templates/navigation/breadcrumb.html.twig

templates/navigation/menu--account.html.twig

templates/navigation/menu--main.html.twig

templates/navigation/pager.html.twig

templates/navigation/views-mini-pager.html.twig

templates/views/views-view.html.twig

theme-settings.php