Make background disabled when overlay is displayed, we should not be able to click on buttons/fill in forms when an overlay (like secret reset, confirmation link) is in front.
The previous attempt resulted in a messed up syling (custom CSS against default topcoat-overlay CSS). Therefore, it make sense to wait for https://issues.jboss.org/browse/AGPUSH-464 to be done, to try out again the topcoat-overlay CSS classes