Remove dotted border on FB.

Some browsers, e.g. Firefox, will add a dotted border to the `abbr` element. This looks really odd in the touch-inspired interface where it is impossible to get to the tooltip.

![Image: screenshot of the site as rendered by Firefox, including a dotted border under the “FB” of “FB message”.](https://cloud.githubusercontent.com/assets/490579/3584091/91c6d0a4-0c13-11e4-84be-bd060e2ccfc3.png)