Spike Solution for Backbone handling 404s

Currently in the application we let Symfony handle 404s, which has pros and cons.

This PR shows how we can let Backbone handle the 404s, and just let Symfony blindly serve the base content.

Advantages:
- Don't have to create a cosmetic route for every URI that we want backbone to be able to hook into

Disadvantages:
- Can't easily generate routes via `Routing.generate` on the client-side, because Symfony would no longer be aware of the route
- Have to serve full client-side app in order to render a 404 view.

@markwilson thoughts on this?