Improve the header navigation style

- The underline only underlines the text, not
  the horizontal padding

- The underline does not overlap with the grey
  border

app/assets/stylesheets/new_design/new_header.scss

@@ -27,12 +27,14 @@
 
 .header-tabs {
   li {
+    @include horizontal-padding($default-padding);
     display: inline-block;
   }
 
   a {
     display: inline-block;
-    padding: 23px $default-padding;
+    height: 71px;
+    line-height: 71px;
     font-size: 18px;
     color: $black;