Merge branch 'header-z-index-fix' into 'master'

Fixed modals being below navbar

See merge request !13355

app/assets/stylesheets/framework/header.scss

@@ -26,7 +26,7 @@ header {
 
   &.navbar-gitlab {
     padding: 0 16px;
-    z-index: 2000;
+    z-index: 1000;
     margin-bottom: 0;
     min-height: $header-height;
     background-color: $gray-light;

app/assets/stylesheets/framework/tw_bootstrap_variables.scss

@@ -162,3 +162,5 @@ $pre-color: $gl-text-color !default;
 $pre-border-color: $border-color;
 
 $table-bg-accent: $gray-light;
+
+$zindex-popover: 900;