use vignette for generating avatar urls

@drsnyder 
Use `VignetteRequest::fromConfigMap` to generate vignette urls for user avatars. We can't use `fromFile` because avatars aren't stored as file objects, they're strings built from the user's id.

Also piggybacking - use `$wgUploadPath` for determining a file's "prefix path" instead of `$wgContLang`. This is needed because some english wikis have their images in `<bucket>/en/images` instead of `<bucket>/images`, and also some wikis have their bucket stored under another, higher level bucket (ex: `p__/psychusa`), so we need to parse that to determine what `parse-prefix` to set for the vignette url.
Just a suggestion: write unit tests for this method.