Clean up z-indexes

There have been some z-indexes that were removed and then reintroduced
through strange merges. This is an attempt to remove all z-indexes that
are unnecessary and to clearly state the purpose of the remaining
z-indexes in a comment to each index.

SPR-981

css/base_structure.less

@@ -66,11 +66,11 @@
 /* + + + + +  z-indeces  + + + + + */
 @media @screen_min-md {
     .nav-direct p {
-        z-index: 1000;
+        z-index: 1000; // keyboard-navigation overlays
     }
 
     .top-header {
-        z-index: 900;
+        z-index: 1; // put MagicMatcher-Dropdowns above .content
     }
 
     .content .row > .col-xs-12 {