Replaced duplicate z-index with index of 400

Former-commit-id: 9afa6684

app/assets/stylesheets/framework/header.scss

@@ -24,14 +24,13 @@ header {
   &.navbar-gitlab {
     padding: 0 16px;
-    z-index: 100;
+    z-index: 400;
     margin-bottom: 0;
     min-height: $header-height;
     background-color: $gray-light;
     border: none;
     border-bottom: 1px solid $border-color;
     position: fixed;
-    z-index: 300;
     top: 0;
     left: 0;
     right: 0;