Making the header width fixed until we have responsive design.

@AlasdairSwan @ndubbaka

FYI @talbs this is due to the following behavior being seen:
![image](https://cloud.githubusercontent.com/assets/235752/4595419/6005c03c-5099-11e4-9f78-ba7403391e30.png)


This was styling that existed in the old header. Probably was a problem then too, but the bigger fonts and buttons probably just makes it stand out more now.

FYI @benpatterson we'll probably want this in before we push to prod.