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_togglelink.less

@@ -12,7 +12,6 @@
         bottom: -1px;
         width: 2rem;
         left: -(@margin-default);
-        z-index: 1;
 
         @media @screen_max-md {
             display: none;