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.
In the case of a `WikiaForeignDBFile` object, yes. This grabs `$wgUploadPath` from that wiki and parses the bucket info from there. In the case of a different language wiki, this will be the same (ex: walkingdead), but in some cases this will be different (ex: shared video thumbnails)