Fix bottom space forgotten when read a message

We forgot to add the bottom space on the page to read a message (http://www.openstreetmap.org/message/read/###). The error can be seen only on long messages.

The value of `$lineheight` is insufficient, but with a value of `30px` is solved.

![img](https://f.cloud.github.com/assets/1282371/1927804/37779d00-7e69-11e3-8b64-cafe0296a68a.png)