v4 alpha 5 - navbar toggler inconsistencies

Created by: niftylettuce

It seems that .navbar-toggler only gets a background-image applied if it is in a .navbar-dark or .navbar-light for example. This doesn't seem right - it should always get a background-image, and there should be a class to turn this off. Otherwise we can't use FontAwesome icon easily with <i class="fa fa-bars"></i>.