Adjust WSGI middleware due to Werkzeug 0.15.0 changes
Werkzeug 0.15.0 changed its default handling of the X-Forwared-Host header, which broke the Flask app's redirects from submitted forms when the app was deployed behind an Apache reverse proxy.
To work around this, we adapt proxied requests using the
werkzeug.middleware.proxy_fix.ProxyFix
middleware.