adding support for lang query param for prefix switching

@drsnyder 
Adding support for lang query param, which will change the prefix when searching for an image. Required because different-language-but-same-wiki wikis (like answers) share the same bucket. Ex:

s3://answers/es/images/0/00/Bella_calamidades.jpg
s3://answers/images/0/00/Bahrain_flag.svg
This also makes me wonder if we should decouple the `:thumb-prefix` and the `:original-prefix` from the `ImageStorage` record. Since those attributes are no longer invariant within a given instance maybe it makes sense to move that logic to the `media-types`.