Don't load oojs-ui.styles on every page
We don't really need it to render the no-JS version
of the notification badges (the only rule we were missing
was background-repeat: no-repeat;) and it significantly
increased the amount of render-blocking CSS.
This means we do need to call setupOOUI() ourselves
in order for our get_class( OOUI\Theme::singleton() ) hack
to keep working.