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.
fixup
Use absolute path in redirect
Merge pull request #881 from nidico/feature-626-nonexistent-subdomains

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