Add default navbar z-index to fix #9844

iOS doesn't pick up the z-indexing properly on the dropdowns unless you
specify one for the parent navbar.

less/variables.less

@@ -188,6 +188,7 @@
 // Used for a bird's eye view of components dependent on the z-axis
 // Try to avoid customizing these :)
 
+@zindex-navbar:            1000;
 @zindex-dropdown:          1000;
 @zindex-popover:           1010;
 @zindex-tooltip:           1030;