Fix video thumbs from video wiki

@drsnyder @garthwebb 
get bucket from the file object, rather than assuming the file always lives in the same bucket as the rest of the current wiki. also, don't replace the thumb url's base domain in ThumbnailVideo, this was causing images with the new path scheme but being sent to the old thumbnailer.
Is the `wgUploadPath` set in `WikiFactory` different from the `$wgUploadPath` global?