Fix overridden padding on email subject

Something in a new version of GOV.UK Elements, Template, or Frontend
Toolkit has introduced a rules which removes padding for the last
column in a table.

This is undesirable in the case of email message previews.

app/assets/stylesheets/components/email-message.scss

@@ -26,9 +26,15 @@ $email-message-gutter: $gutter * 2;
     }
 
     td {
+
       width: 99%;
       padding-right: $email-message-gutter;
       word-break: break-word;
+
+      &:first-child {
+        padding-right: $email-message-gutter;
+      }
+
     }
 
   }