When proxied, don't show banners unless proxy is known to be zero-rated.

* The canonical X-Forwarded-By header will be used to determine any proxy.
* If the proxy is known to be zero-rated for a carrier, the banner will be shown.
* If the proxy isn't known to be zero-rated for a carrier, the banner will not be shown.
* If there is no X-Forwarded-By header, normal banner display rules will be used.
* See also I5120597b30eafddc3dbd434fb8d55bee82c47295.

Change-Id: I9046ff0044ae8b722aab499fd89c799dc971dd6e