Add flash containers and broadcast messages below subnav

CHANGELOG

app/assets/stylesheets/framework/flash.scss

app/views/admin/background_jobs/_head.html.haml

app/views/admin/dashboard/_head.html.haml

app/views/layouts/_flash.html.haml

app/views/layouts/_page.html.haml

app/views/projects/commits/_head.html.haml

app/views/projects/commits/show.html.haml

app/views/projects/graphs/_head.html.haml

app/views/projects/issues/_head.html.haml

app/views/projects/issues/index.html.haml

app/views/projects/pipelines/_head.html.haml

app/views/projects/wikis/_nav.html.haml