[LESS] [content] lower content render order

Create a new stacking context for page content. Previously, it was
possible for on page content to defeat the skin's z-index values with
superior numbers. This occurred with StructuredDiscussion OOUI widgets
on https://test.m.wikipedia.org/wiki/Talk:Main_Page and the
`position: sticky` header on Special:Notifications.

`scaleY(1)` is an identity transform that is terse and unlikely to be
confused when RTL flipping.

Bug: T225959
Change-Id: If8f718a707d9dd07e1182bc26e63e6e665bf98c2

resources/skins.minerva.base.styles/common.less

@@ -44,5 +44,10 @@
 	width: 100%;
 	position: relative;
 	background-color: @chromeColor;
+}
+
+.content {
+	// Create a new stacking context.
+	transform: scaleY( 1 );
 	z-index: @z-indexBase;
 }