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/area_nav-metabox.less

@@ -12,7 +12,7 @@
     .justify-content(flex-end);
 
     position: relative;
-    z-index: 10;
+    z-index: 1; // put meta-box above positioned content-elements such as aggregations, edit-buttons
     float: right;
     max-width: 40%;
     margin-top: -(@page_padding-top); // reverse padding-top of .page container
@@ -158,7 +158,7 @@
             }
 
             &.active {
-                z-index: 1;
+                z-index: 1; // put the active tab above the meta-content in .tab-pane.active
 
                 > a {
                     cursor: default;