Add CSS to fix alert icons - #457

This Pull Request fixes the icons in the warning boxes. Currently the CSS on the before pseudo element has the styles

height: 1rem;
width: 1rem;
content: "";
height: 2px;
background-image: url(../img/usa-icons/warning.svg);
background-position: 50%;
background-repeat: no-repeat;
background-size: 1rem 1rem;
display: block;
position: absolute;
top: .5625rem;
left: 1rem;

With height: 2px taking precedent over the height: 1rem. I'm re-adding the height: 1rem with higher precedent to "win."

I know we don't like to add CSS directly, but I can not find where the height: 2px is being added to the element's style.

Before:

After: