Using box-shadow for link focus to stop links from shifting a few pixels when tabbed.

(CC @marcelgerber.)
Yeah if there's no clear advantage of using outline instead of box-shadow I'd rather not add an extra property.

Focus ring on "Update" looks like this here:
![screen shot 2014-08-26 at 1 13 08 pm](https://cloud.githubusercontent.com/assets/1495261/4050978/6f799ade-2d5d-11e4-96a6-aa818df15e7d.png)