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.

Addresses https://github.com/hypothesis/h/issues/480
Can you add a comment like `# Dispatch ContextFound for pyramid_layout subscriber`?