404.html file not being served

Currently the `404.html` file in the `app/views` folder can't be served as express is sending `index.html` for any request that isn't an API call or a partial.

This PR uses the same methodology as for the partials - i.e. first express tries to render a file by the passed name, otherwise falling back to `index.html`. This way, custom error pages and any other static / server-side pages can be added to the `app/views` folder and served as normal.