mediawiki.notification: Also hide #mw-notification-area upon creation

mediawiki.notification: Also hide #mw-notification-area upon creation

6c5b246 made it so #mw-notification-area was hidden when the last
notification was dismissed, but it still left it visible when
initially creating it. This meant that if you used mw.notify(),
you had no problems, but if you loaded the mediawiki.notification
module manually and called it manually, you would still get
a 12px-tall click-stealing invisible div until the first notification
was displayed.

Bug: 52659
Change-Id: Ibec399ac9ef083036aa2b31a38bcc1bc36b87d96