X-Forwarded-For with proxy addresses is not handled properly

[Scrin]

It looks like if one uses a reverse proxy in front of MMR that includes X-Forwarded-For header with possible proxy address(es), MMR does not handle that properly. I believe the issue is here:

matrix-media-repo/api/_routers/03-host_detection.go

Lines 37 to 45 in 85fabea

	if config.Get().General.TrustAnyForward {
	raddr = r.Header.Get("X-Forwarded-For")
	} else {
	raddr = xff.GetRemoteAddr(r)
	}
	if raddr == "" {
	raddr = r.RemoteAddr
	}
	host, _, err := net.SplitHostPort(raddr)

If the raddr gets its value from the X-Forwarded-For and the header contains proxy addresses (see https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For#syntax ), then the net.SplitHostPort fails to parse it

[Scrin]

I can fix this, but I'd like to see #611 merged first to avoid conflicts, as mentioned in the MMR matrix room