Add higher z-index to fixed nav

Former-commit-id: 912d02bf

app/assets/stylesheets/framework/header.scss

@@ -31,6 +31,7 @@ header {
     border: none;
     border-bottom: 1px solid $border-color;
     position: fixed;
+    z-index: 300;
     top: 0;
     left: 0;
     right: 0;