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/magic-matcher.less

@@ -47,7 +47,6 @@
 
     #magicmatcher__context {
         position: relative;
-        z-index: 100;
         width: 100%;
         min-height: @height-context-bar;
         box-sizing: border-box;