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/235752/4595403/2506fcee-5099-11e4-9f78-ee63572a3df0.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.