[bug 929647] Fix throttling to use correct ip

This fixes an egregious error I made where the ip address was being
incorrectly pulled from REMOTE_ADDR but should be pulled from
HTTP_X_CLUSTER_CLIENT_IP because our servers are behind a reverse proxy.

This does what Kitsune does here (minus the username-or-ip part): https://github.com/mozilla/kitsune/blob/master/kitsune/sumo/utils.py#L158

Quick r?