Fix for progressbar styling

Modified the styling for the progressbar as the last fix didn't seem to
work in latest updates of Chrome, where the progressbar in Umbraco 7.1.6 and Chrome Version 37.0.2062.103 m hadn't a full width.

Previously pull requests: https://github.com/umbraco/Umbraco-CMS/pull/420 and https://github.com/umbraco/Umbraco-CMS/pull/456
I'd like to see this styles as part of the css/less files instead of inline styles. It's ok to hack inline styles in the legacy webforms stuff but we need to keep the new angular files clean.