Allow fallbacks for $wgReferrerPolicy

Allow fallbacks for $wgReferrerPolicy

For browsers that support older versions of the Referrer Policy
specification (Edge and Safari), using a value from a newer version of
the specification will result in those browsers falling back to a
default of "default".

So allow $wgReferrerPolicy to have fallbacks if the browser does not
recognize the currently set value. It will emit <meta> tags for each
value in the array, but in reverse order, as browsers will use the last
one that they recognize.

Bug: T180921
Change-Id: Ie0f523fc6937c9ecffc8a6fc791c6b54d5a1cb06