Prevent editing textarea from jumping when toolbar is loaded

Prevent editing textarea from jumping when toolbar is loaded

If you're using the "classic" editor (not the WikiEditor editor),
the editing textarea jumps down when the toolbar loads, which can
take a few seconds on a slow connection. This can cause editors to
accidently click the toolbar instead of the editing textarea and
accidently insert markup into the article that they didn't mean to.

This rule has been on English Wikipedia's Common.css since 2007.

Change-Id: Ie4ac15c95554608c838ef0e112924fc6381a6f66