custom wordmark urls

@drsnyder @owend 
https://wikia-inc.atlassian.net/browse/PLATFORM-561

Looks like custom wordmarks saved while having Vignette enabled get saved correctly, but there is some url rewriting that goes on when the image is set to get displayed. This fix actually required detecting if a URL is a Vignette URL, since `$wgEnableVignette` doesn't mean the URL was saved into WikiFactory as a Vignette URL.
@macbre Do you see any issue with using the File object for `wordmark-image-url` instead of the URL string?