Hide Extra-Long admin bar text

[filosofo]

Each admin bar link has a fixed height and width (thanks to the requirements of the background image sprite), so it has white-space:nowrap set.

That means that with a few words, the text hangs hideously outside the box of the admin bar dropdown.

Patch adds the overflow:hidden rule so it disappears politely at the edge of the list item.

Please see the attached before and after screenshots for a demonstration.

(In [16664]) Hide Extra-Long admin bar text. Props filosofo. fixes #15412