Add header variable for z index

app/assets/stylesheets/framework/header.scss

 .navbar-gitlab {
   padding: 0 16px;
-  z-index: 1000;
+  z-index: $header-zindex;
   margin-bottom: 0;
   min-height: $header-height;
   border: 0;

app/assets/stylesheets/framework/variables.scss

@@ -386,6 +386,7 @@ $browser-scrollbar-size: 10px;
  * Misc
  */
 $header-height: 40px;
+$header-zindex: 1000;
 $suggestion-header-height: 46px;
 $ide-statusbar-height: 25px;
 $fixed-layout-width: 1280px;

app/assets/stylesheets/pages/editor.scss

@@ -223,5 +223,5 @@
 }
 
 .popover.suggest-gitlab-ci-yml {
-  z-index: 240;
+  z-index: $header-zindex - 1;
 }