fix(core): 🐛 explictly set z-index for body container

That should avoid position relative elements from going over the sticky header.

Closes: #577

resources/skins.citizen.styles/layout.less

@@ -47,6 +47,7 @@
 }
 
 .citizen-body {
+	z-index: @z-index-base; // Stop body content going over sticky header
 	grid-area: content;
 }