Unbreak 301 responses

Unbreak 301 responses

The move to werkzeug routing went pretty smooth, but one thing was
broken by accident: URLs without final slash result in a 301
werkzeug.routing.RequestRedirect response. We displayed it as a generic
error page rather than actually sending the redirect. Do that. One thing
it does though is to skip all meddlewares, which should be OK for a 301
response, but might need rework if we decide otherwise. With this, 301
responses with lacking final slash are unbroken again.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>