Add 404 page

This PR adds a basic 404 page:
![page_not_found](https://cloud.githubusercontent.com/assets/521978/4710054/1c535ef8-58a5-11e4-8855-dc9c182ff48a.png)

A few things to note:
* I looked at quite a few 404 pages for reference and I notice many of them included search. Entering a query into the search box on our 404 page will search the stream.
* For now, we've left the 404 branding neutral since this 404 page will also be shown in places like [webplatform.org](http://notes.webplatform.org/).
* Eventually, it would be great if we could have the topbar be present as on /stream or the standalone view, with the user logged in and able to search, log out, read notifications, etc. Ideally, this is where branding information would also be displayed.