Custom 404 page.

[AlexanderOtavka]

We should have a custom 404 page that is interesting. Just put it in app/404.html and GitHub will serve it if a 404 happens.

[SebastianBoyd]

No longer needed, 404s redirect to homepage

[AlexanderOtavka]

The 404s redirecting to homepage is a weird behavior of the gulp server. We could still use a custom 404 page for the live version served by GitHub.