Support setting a separate referrer policy for Maps

[pskyechology]

If a wiki has their referrer policy set to no referrer, then they can have issues with Leaflet tiles returning errors due to violating OSM's Tile Usage Policy.

To avoid this from happening, Maps should allow for setting a separate referrer policy for individual calls or extension-wide.

[krabina]

As a workaround, this can be used
$wgReferrerPolicy = [ 'strict-origin-when-cross-origin', 'no-referrer-when-downgrade' ];