Use leftmost address when parsing `X-Forwarded-For` header for client IP

When a client passes through multiple proxies, each subsequent address is appended to the X-Forwarded-For header, resulting in a comma-separated list of IP addresses:

X-Forwarded-For: <client>, <proxy1>, <proxy2>

Right now BridgeDB only looks for the client's IP in the rightmost address

    if useForwardedHeader:
        header = request.getHeader("X-Forwarded-For")
        if header:
            index = -1
            ip = header.split(",")[index].strip()
            if skipLoopback:
                logging.info(("Parsing X-Forwarded-For again, ignoring "
                              "loopback addresses..."))
                while isLoopback(ip):
                    index -= 1
                    ip = header.split(",")[index].strip()
            if not skipLoopback and isLoopback(ip):
               logging.warn("Accepting loopback address: %s" % ip)
            else:
                if not isIPAddress(ip):
                    logging.warn("Got weird X-Forwarded-For value %r" % header)
                    ip = None

This causes trouble with our Moat and Apache ProxyPass setup, which results in X-Forwarded-For headers like the following:

X-Forwarded-For: <client>, ... <proxies> ... <local address>

I think we should modify this to parse the addresses from left to right, ignoring loopback/internal addresses, until we find a valid address for the client.

This is a follow-up modification for #32276 (closed) and a prerequisite for #40025 (closed).