mediawiki.notification: Use css() instead of slow show()/hide()

mediawiki.notification: Use css() instead of slow show()/hide()

There is a lot of complexity and overhead involved with show()
and hide() methods for back-compat and edge cases that we don't
want to involve here (such as storing the last known display,
figuring out the default display, dealing with elements hidden
by default via CSS rules etc.).

We know the stylesheet does not hide the element by default, so
we merely need to set or unset the 'display: none;' inline style.

Ref T159911.

Change-Id: Ia22a2adddddf8514368124d93e3a753440814d97