[dev] [LESS] use z-index symbol instead of hardcode

@z-indexBase is 0. @z-indexBase - 1 is -1 which is identical to the
symbol @z-indexOccluded. The calculation didn't seem to add much to the
clarity of the code so use the symbol for grepability.

Change-Id: Ie8dcbca00a0a3e1d08a4ffe0b4fb3b71257f69c0

resources/skins.minerva.content.styles/templates/ambox.less

@@ -119,7 +119,7 @@ table.ambox {
 		&:before {
 			content: '';
 			position: absolute;
-			z-index: @z-indexBase - 1; // set z-index 1 below the parent element.
+			z-index: @z-indexOccluded;
 			bottom: 0;
 			right: 0;
 			width: 100%; // width & height defined by length of "learn more".