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/plugins/do_tasks.less

@@ -95,5 +95,5 @@ ul.page-attributes {
 }
 
 .plugin__do_usertasks_list {
-    z-index: 5;
+    z-index: 200; // put tasks-list above pagetools
 }