Vector: Use .mediawiki instead of .skin-vector in notification styles

What was I thinking when I did this in I2b3d9cbc?

The extra class is present as mediawiki.notification's css is loaded later
thus overriding the skin's css. However using .skin-vector for that
purpose breaks Vector derived skins since they don't have it. The .mediawiki
class is always present so it should be an acceptable weight.

Change-Id: Ica2a3d3a2ef25aabb38fac36e9eaf142c8bbd388