Proper treatment of non-existent subdomains (fixes #626)

This commit does two useful things at once, I'm too lazy to artificially
split them up:

1. Show proper 404 instead of white page for non-existing URLs (this is
#626). This was due to the StatusCodeRedirect middleware running into
the same exceptional case in the InstanceDiscriminatorMiddleware again
when trying to resolve http://nonexistent.adhocracy.lan/error/document
and is fixed by setting the HTTP_HOST environment variable to the base
adhocracy domain.

2. Prefixing a non-relative_url installation with www we're now being
redirected to the real URL, i.e. the corresponding path in the main
domain instead of silently defaulting to that resource.