Normalize interstitial for images.

* Currently, Wikipedia Zero users on carriers who do not whitelist images end up at an interstitial on a File:-namespaced path.
* This change sends the user to the standardized interstitial based on the page from which the image is hyperlinked.
* This will prevent commons.wikimedia.org stylesheets for File:-namespaced requests from unintentionally being loaded for the common case of a user clicking on the "Click to view" link prior to actually accepting the charge warning.

Change-Id: I3d8f9333df97f2ea962d2678dcb3bebb68f0c7b2