Add trust_proxy_headers middleware

[aantron]

...which would allow opting in to, one by one, trusting:

• X-Forwarded-Host, Forwarded — likely needed for Dream.origin_referer_check AJAX CSRF protection.
• X-Forwarded-For
• X-Forwarded-Proto — this is needed for correctly inferring cookie Secure attributes when working behind a proxy.
• Some request ID header, to use an extrenal ID.
• Create an nginx example?