Hack the header to make sign-in links not completely broken on mobile

See https://github.com/codeforamerica/bizfriendly-web/issues/138

I don't think it's an ideal solution. But at least it looks okay?

![image](https://f.cloud.github.com/assets/2553268/1882289/07c4c2ee-7974-11e3-8ea0-7b355540a753.png)