WikidataPageBanner: PAGEBANNER magic word added

'PAGEBANNER' magic word added to WikidataPageBanner extension, which accepts an
image file url on the wiki and renders an alternative pagebanner with that image
as the background. Allows the user to control banner image.

Bug: T97455
Change-Id: I37d02267b61fc070b85a9974eefc5cb66b81083e