Use box-sizing:border-box for textarea

Use box-sizing:border-box for textarea

Bug 40610 identified an overflowing textarea for #editform and fixed this.
All textarea elements overflows because they use the following CSS
definition from skins/common/commonElements.css:

textarea {
    width: 100%;
    padding: .1em;
}

In standard compliance mode of the box model this generates a width
of 100% plus .1em for the border box. To create a border box with a
width of 100% definition box-sizing:border-box should be added:

textarea {
    width: 100%;
    padding: .1em;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
}

Bug: 56692
Change-Id: Ife1cf74ca33b4f53ec052947bd8b7b2fd28705ce