Fixed misplaced cursor in IE8

The CSS reset in core/js/styles.css defines font-size: 100% on most
elements. This messes up the font size calculations of ACE in IE8.

This IE8 specific fix overrides the font-size using a fixed size to make it work.

Fixes https://github.com/owncloud/core/issues/5051

@jancborchardt @tomneedham @karlitschek @kabum