Show 404 page when no user can be found

closes #96 

It can definitely use more styling, or something fun, as a good 404 site should have. I still think this is better than 502 ^^

To test locally, and not see the standard django Page Not Found, you have to set `DEBUG = False`. This might render a 500 error. In that case, add `ALLOWED_HOSTS = ['localhost']` to your local.py.
For extra fun this will cause all static files to fail loading, so you only actually get a preview of the html.
I guess one way around this is setting up a temporary custom view that renders the 404.html template.