[SCSS] FIX - appearence none

Created by: nicovak

There is an issue with select and appearance css property. The dropdown 'icon' is displayed twice (tested on chrome). A mixin should be used (It's SASS), Bourbon is imported, you could used it like this:

Eg: administrate/app/assets/stylesheets/administrate/base/_forms.scss

select {
@include appearance(none);
}

It will generates all browser's prefix for the property.

IE is the bad kid but Opera seems to be also : W3C

The best way to hanlde It would be to include a javascript library for a better customization.