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/template_detail.less

@@ -24,7 +24,7 @@
                 position: absolute;
                 top: 0;
                 left: 0;
-                z-index: 2;
+                z-index: 2; // put 'view original file'-overlay above image
                 display: block;
                 width: 100%;
                 box-sizing: border-box;
@@ -38,7 +38,6 @@
                 display: block;
                 border: 1px dotted @ini_background_site;
                 position: relative;
-                z-index: 1;
             }
 
             &:hover,