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.
We talked about this in chat-- It looks like what is being stored is the URL to the wordmark and not the File object. The URL is derivative which is why we are having to do this URL detection logic. This is intended as a hot fix. We're filing a ticket to come back and have the File object stored instead of the URL.