Make calling wfMangleFlashPolicy configurable

Add $wgMangleFlashPolicy so sites that define a safe master flash
policy can disable mangling of the <cross-domain-policy> string.

Bug: 66776
Change-Id: Ic0db939aae9d0bb6bdfc3cd9ce282661bad46783
(cherry picked from commit 92f22cd466e2e20bf0ba6947abaa256b922c193d)